EARLY REAGAN By Anne Edwards Morrow. 617 pp. $21.95

SINCE Ronald Reagan has been at it now for nearly seven years, Americans, if not the rest of the world, have gradually gotten used to the idea that the highest office in the land is really and truly occupied by a former movie actor. What would have seemed unthinkable 40 years ago has come to pass with a smooth sense of inevitability that would have left Wendell Willkie, Thomas W. Dewey and other conventional Republicans of that earlier age gasping in astonishment.

Well, in spite of worst fears and dire predictions, the Republic has held. A year and a half out of port, the ship of state sails on. It looks as if Reagan may make it.

Nevertheless, when P. J. O'Rourke, Christopher Buckley or one of their friends comes to write the three-volume biography of the 40th president of the United States, he will find that the question that must be answered in volume one is just how Ronald Reagan managed to make it from B-movies to the Governor's Mansion in California -- one that still puzzles a lot of Democrats. But luckily for him, a good deal of the grunt work has already been done here, and very well, by Anne Edwards in Early Reagan. In it, she covers the first 55 years of his life; the book ends with his announcement on Jan. 4, 1966, that he would run for Governor of California. Because he worked in films and television for 30 years before that, Early Reagan is perforce a Hollywood biography. Edwards has written them before: Judy Garland, Vivien Leigh and Katharine Hepburn were her subjects in the past (as well as Queen Mary, Sonya Tolstoy, and Margaret Mitchell). But for a couple of reasons this one is quite unlike any Hollywood biography ever done before.

First of all, you can be sure that no other actor with such a mediocre career would have rated such treatment. What is most remarkable about Reagan's professional life is that it was both so long and so undistinguished. In 30 years he never won an award, never even received a nomination. By Anne Edwards' estimate, in a career of 53 films he made only four that were "truly prestigious" -- Dark Victory; Knute Rockne, All American; King's Row; and The Hasty Heart. He started in Warner Brothers' B-unit, the most eager and cooperative of Bryan Foy's charges (he never turned down a part), and he set his personal artistic standard with movies like Secret Service of the Air and Tugboat Annie Sails Again. He wasn't really interested in acting in the same way that his first wife, Jane Wyman (Johnny Belinda, The Yearling) was. The movies gave him money and celebrity, and that seemed to satisfy him. When, in the '50s, the film assignments began to peter out, he was more than willing to accept a deal in television which, at the cost of some loss of prestige, made him the host (and later producer) of G.E. Theater, as well as corporate representative for the sponsor, General Electric. The money was right.

THAT JOB with G.E. is generally cited as the informal beginning of Reagan's politicial career. He was so successful speaking for the company to unions, communities, and to the public at large that he was told, "You're not our spokesman, even though you're going out under our aegis. You're speaking for yourself. You say what you believe." Eventually, the liberal wing of G.E. management had reason to regret the free hand he had been given: Reagan's brand of Americanism was too conservative for them. And to think he started out a New Deal Democrat!

In this way, too, Early Reagan differs from all other movie biographies, for Edwards never forgets that she is dealing with a politician, albeit a strange one, and she manages with some precision to trace the shift in his political thinking from left to right. As early as 1939, after many political arguments, ardent Republican Dick Powell told Reagan that if he switched parties he would find financing for a campaign to elect him to Congress -- but Reagan held firm. It was during his years as president of the Screen Actors Guild, when he showed himself a passionate anti-Communist and a staunch defender of the movie blacklist, that the change seemed to take place. (He even became an FBI informant then.) In fact, Edwards places his shift in allegiance 12 years earlier than the date he has given out, 1962. While officially supporting Helen Gahagan {ck bk for sp.} Douglas in her campaign for the Senate in 1950, Reagan was secretly working for her opponent, Richard Nixon.

Finally, it remains to be said in praise of Early Reagan that it offers the most complete account to date of Ronald Reagan's pre-Hollywood years. Edwards covered his tracks thoroughly in Dixon, Ill., where he grew up; at Eureka College, where he proved more interested in athletics than his studies; and in Des Moines, where he became a local celebrity as a sportscaster and interviewer. There is no hint of condescension in Edwards' treatment of this material; his mid-American background may not be hers, but she treats it with respect. She interviewed widely and in some depth, and the picture of young Reagan that emerges is of the kind of young American who believes profoundly in the traditional values with which he was reared, who is determined to succeed, the kind who might indeed become president of something someday. But of the United States?

From time to time, the book does seem over-researched and a bit overwritten. There are, as well, a few small errors of fact. For example, it was William Cullen Bryant and not James Russell Lowell who wrote "To a Waterfowl"; Thousand Oaks, California, is located in the Conejo Valley and not the San Fernando; and although Sam Rayburn served in Congress many years, he was never a Senator. A good editor would have caught such slips as these and would also have persuaded her to cut 25 to 50 pages from her backgrounding digressions. All that this amounts to, however, is a half-hearted complaint that a good book -- one written with sympathy, understanding, yet a healthy critical spirit -- could have been a little bit better.

Bruce Cook is the author of "The Beat Generation" and "Brecht in Exile."