David McCullough specializes in writing books that can't be made into movies. Who, after all, could afford the vast period sets and huge casts required to film the building of the Brooklyn Bridge or the Panama Canal?

Well, as it happens, producer George Englund plans to turn McCullough's "The Great Bridge" into a TV movie to coincide with the Brooklyn Bridge centennial in 1983, and "The Path Between the Seas" is to be a 10-hour, $20-million TV mini-series, produced by Carlo and Alex Ponti. The author has been back to Panama recently, scouting locations, and he has completed an outline -- to which the producers' only objection was that it would take 20 hours to dramatize, and cost $40 million.

With his two other books -- "The Johnstown Flood," and the just-published "Mornings on Horseback" about Theodore Roosevelt -- McCullough's work adds up to a composite study of American daring, enterprise and persistence. The same qualities, on occasion, have been demanded of the author. In 1974 McCullough found the French phase of the canal story -- the 15-year period when Ferdinand de Lesseps hoped to cut through Panama as he had cut through Suez -- looming larger and larger in the scheme of his book. Like the canal excavation itself, which wound up consuming six times the volume of earth originally estimated, the scope of his research kept growing as each discovery led him deeper into the source material.

"Three years went by and I was only about half finished," says the youthfully gray, 47-year-old historian.

His $50,000 advance was all finished, however, and there were friends who wondered how a mammoth history of such a remote event could ever repay the author's time. "Who'd want to read a book about the Panama Canal?" they said. The author himself made so bold as to predict that the canal issue "would blow sky-high' sooner or later, but he never suspected that the publication and the explosion would come simultaneously -- that "The Path Between the Seas" would be a Book-if-the-Month-Club main selection and win every prize in sight, that then-president Jimmy Carter would stay up until 4 a.m. one night reading it, and that it would be credited with helping a new canal treaty work its way through a grudging U.S. Senate.

McCullough's books are filled with portraits of "people struggling against great odds to accomplish great objectives," and people who have been ignored or trivialized by previous historians. "I get a huge kick out of writing about people who haven't gotten a fair shake," he says. Trials of the Bridge

Two such people, until McCullough came along, were John and Washington Roebling, the designer and engineer of the Brooklyn Bridge. John Roebling was a German immigrant who invented cable -- "iron rope" -- in 1841, and became the foremost bridge builder of his day. In 1869, part of his foot was crushed during the early stages of the Brooklyn Bridge's construction, and a few weeks later he contracted tetanus and died a horrible death. The bridge would ruin his son's health, too. Washington Roebling, like many others who toiled in the caissons under the East River, was crippled by the mysterious disease called "the bends." But for 14 years he would battle the graft, the engineering problems and the medical calamities that repeatedly threatened the "eighth wonder of the world."

"I think the most phenomenal thing about the Brooklyn Bridge is that today it works," says McCullough. "And we're in an era when practically nothing works. They were building the bridge to last forever."

To this day, the bridge contains faulty steel cable supplied by a corrupt manufacturer, who won the contract in part because critics questioned the seemliness of giving it to the Roeblings themselves. J. Lloyd Haigh, the successful bidder, would submit a worthy sample of cable to the bridge inspectors, then subtitute the inferior goods on the way to the construction site, and then repeated the whole routine with the next shipment. The scam wasn't exposed until it had continued for months, but the bad cable was left in place because, true to form, the Roeblings had designed the bridge to be eight times stronger than necessary. That also explains why a bridge built two decades before the invention of the automobile now carries 121,000 trucks and automobiles a day.

The durability of the Brooklyn Bridge is a monument to "the integrity of the builders," says McCullough. "John Roebling was a genius. Washington Roebling was a hero -- a hero in the classical sense, who had one parent who was a god."

Two more of McCullough's unsung heroes are William C. Gorgas, the Army physician who succeeded in virtually eliminating malaria and yellow fever from the Panama Canal Zone despite stultifying bureaucratic indifference and second-guessing; and John Stevens, who rescued the canal project after a disastrous year of pettiness and mismanagement, accompanied by death on a grand scale. "There are three diseases in Panama," Stevens declared soon after taking command. "They are yellow fever, malaria and cold feet, and the greatest of these is cold feet."

McCollough's own feet could have gone cold during the writing of his latest book. When the first volume of Edmund Morris' "Theodore Roosevelt" came out two years ago, McCullough's friends "would call me like there had been a death in the family," he says. At the time, he was deeply immersed in researching some of the same years in the life of the same man. McCullough had begun "Mornings on Horseback" in blissful ignorance of Morris and his project. Later he knew only that a British journalist who had never written a book was doing a life of Roosevelt. So it must have been a surprise when Morris' work mowed down the critics and then the arbiters of the Pulitzer Prize for biography. And there must have been some doubt about whether the market would bear two books on such a subject in such short order.

But McCullough says he was never worried. To begin with, he doesn't regard his book as a biography. It is a protrait, he says, of a well-to-do, civic-minded, 19th-century American family, and a study of how one extraordinary man emerged from that family. Besides, he says, "the joy in my work is doing it. I had the most wonderful time doing it." Stumbling Into History

The man who talks about his work with such exuberance didn't think of writing history for a living until he was 31. He had majored in English at Yale. "In fact, I've never taken an American history course except in high school," he says. From 1956-'61 he worked for Time-Life, first in the Sports Illustrated circulation department and later as a writer with Architectural Forum. Then he came to Washington and the U.S. Information Agency, to edit a USIA-sponsored magazine for the Arab world. It was on USIA business that he came across a photograph of the opening of the Statue of Liberty -- a picture which so impressed McCollough that he took a copy to the offices of American Heritage magazine as something that might be worthy of publication. "And they said why didn't I write a caption to go with it and we'll pay you a finder's fee for the picture."

The caption ran 20 pages, and a historian was born. In 1964, McCullough left Washington and the government for New York and an editor's job at American Heritage. He wrote his first book, "The Johnstown Flood," in his spare time, and despite an unglamorous-sounding subject, it sold well enough to encourage him to abandon salaried employment completely in 1970.

McCullough has spent most of his recent life burrowing in the 19th century, and his books could give aid and comfort to those who feel the 20th century hasn't measured up. "One of the notions that was prevalent in the 19th century was that people ought to be useful," says McCullough. "That's a very different attitude than doing your own thing or the hedonistic preoccupation with self that has become part of the psycho-blabber of now . . . pHappiness and ease are not synonymous. They understand that, I think, more than we do now." He also points out that "the most important tonic in society is that belief that tomorrow can be made better than today" -- a belief that was almost universal in the 19th century, and rarely heard nowadays outside the walls of the Office of Management and Budget.

But McCullough has a caution for 19th-Century-worshippers. "I'll tell you what I don't like about the 19th century. I don't like their racism. I don't like some of their sentimentality, their heavy religiosity. Any century that could go for Henry Ward Beecher can't be all good, and I dispair, as anyone would, about the real exploitation of working people."

His books are curiously interconnected. The first use of iron cable, after John Roebling had conceived and fashioned it in 1841, was to haul canal boats over the Allegheny Mountains at Johnstown, Pa., scene of the flood. A generation later, Roebling cable went into the steam shovels that excavated the Panama Canal, and the canal came to be built only when Theodore Roosevelt took the extraordinary step of actually listening to the experts rather than submitting to massive political pressures that favored a potentially more troublesome route through Nicaragua.

Roosevelt was also, of course, a key finagler behind one of the more carefully stage-managed events in the history of political upheaval -- the Panamanian Revolution. Perhaps more than anyone, Teddy Roosevelt symbolizes all the contradictory qualities of 19th-century America -- jingoism, idealism, scholarship, dilettantism, ruggedness and luxury. He has been badly misunderstood, according to McCullough. Most Americans (particularly if they have seen "Arsenic and Old Lace") have a cartoon vision of Roosevelt riding up San Juan Hill shouting "Bully!" If they know anything about his private life, it is that he "conquered" his asthma and general childhood frailty through brutal exercise, a notion McCullough thinks historians and biographers (Morris included) have been far too ready to accept. The Roosevelt Research

For "Mornings on Horseback," McCullough filled his head with information about asthma, just as he had with information about the bends for "The Great Bridge," and malaria and yellow fever for "The Path Between the Seas."

Roosevelt suffered from terrible asthmatic attacks until his college years. "If you don't understand the hell he went through as a child, you can't understand the dimensions of his success as an adult," says McCullough. cWorking with diaries from a part of Roosevelt's childhood when every asthma attack was recorded, he noted them on a calendar and tried to look for a pattern. "I was coming at it [the subject] from every possible angle and I was getting nowhere," he says.He looked for links with seasons, places and people. "I don't know how many times I looked at that thing, and I never saw it." Then he did, and "I can tell you, honest to God, it was just exactly like an electric shock. All the red entries were right down the left-hand side of the page. He was having his attacks on Sunday."

Why Sundays? It was his father's day home from the office, when, in the ordinary course of events, the entire family would share their patriarch's company. When Teddy suffered one of his attacks, however, "the payoff was a day in the country with father -- just the two of them," says McCullough. In general, he says, the asthmatic "wants to be the bride at every wedding and the corpse at every funeral." Teddy Roosevelt was never "cured," but the disease abated as he grew older, and particularly when he left home and family for Harvard and the world beyond. It wasn't the exercise that conquered the problem, McCullough theorizes, but Roosevelt's success in finding "a larger stage to perform on."

Along with this venture into the treacherous territory of psychobiography, McCullough tried to look more deeply than other writers into Roosevelt's family. "I have a feeling that much of what is called biography is a con, in that the writer, in order to sbow the growth of his main character, gives the supporting cast as stereotypes," he says. Toward that end, he read the surviving correspondence not only to and from Roosevelt himself, but among various other members of the family. "Most of the correspondence that I've worked with, people haven't taken the time to look at," he says, and because "it predates Freud and self-consciousness, there are things in those letters that people would never say today."

While at Harvard, Roosevelt would address his mother as "my own sweet Motherling" and his younger sister Corinne as "Little Pet Pussie." "I want to pet you again awfully!" he wrote her. "You cunning, pretty, little, foolish Puss. My easy chair would just hold myself and Pussie." People of that era "wrote all the time about what they were doing," says McCullough wistfully. "Those people wrote letters two or three times a day . . . Nobody's going to be able to write books [on current subjects] like I'm writing because we don't leave any records."

He's not worried about running out of material for further books, however. "The number of subjects that have either been not done or done badly is just astonishing," he says. Just by example, he points out that there has never been a biography of John Roebling, "one of the looming figures of the 19th century." (But McCullough is not about to fill the gap; he says his next book will deal with a more contemporary subject.)

McCullough works in an 8-by-12-foot shingled study, with no telephone or plumbing, 100 yards away from his home on Martha's Vineyard. "If I had it to do over again, I would certainly build it larger," he says.

He tries to start around 8:30 a.m., and to stay with it until 5:30 or 6 p.m., except for an hour's break to eat lunch and "look at the mail and visit with my wife." His research is organized into notebooks devoted to characters and/or themes, with handwritten notes and photocopied matter interspersed. He gets much of the material -- books and microfilm -- from inter-library loan, but tries to acquire his own copy of a book whenever possible because that it pays to buy the book." When the material won't come to him, he goes to the material -- which, in the case of Teddy Roosevelt, meant to Harvard, where the Roosevelt collection is housed. The Ink Blot

He has made frequent trips to Washington, including one, a year ago, to look at an ink blot. Not the Rohrschach kind but the Roosevelt kind. During his junior year at Harvard, Teddy Roosevelt had smeared over a whole page of his diary with ink. A microfilm copy of the diary was in Harvard's Lamont library, but McCullough wanted to see the original. "I thought to myself, 'My God! I'd like to see that ink blot!'" So he went to the Library of Congress, and then he went to a library employe with a question. He said he thought it would be nice to see what was under the ink blot. The employe agreed that, yes, of course it would be nice, but it woud be highly irregular, if not absolutely unheard of, and if McCullough was really serious, well, perhaps there was a higher-up who could talk to him.

Then John Knowlton, a higher-up, was summoned, and as soon as he saw the page, he saw the point. "Yeah, that would be great," he said.

McCullough had thought of the FBI, reasoning that if that diary were part of a criminal investigation, the bureau would find a way. So the page was sent to the FBI, and a few weeks later it came back, undeciphered and certified hopeless.

That's when Knowlton and McCullough got serious. They found a photographic process that would separate the layers of ink, but the blurry image of the original notations wasn't easy to read, and days of intensive paleography passed before these words were finally deciphered:

"Angry with myself," the young Roosevelt had written, "for getting tight."