What's new in American publishing? Virtually nothing, to judge from "A Nation of Readers," the small wonder of an exhibition that opens today at the Library of Congress. Among the more than 100 books and other objects tracing Americans' buying habits from the late 17th century to the present, the delighted browser will discover the ancestors of many a modern trend:

Self-help books? Eat your heart out, Wayne Dyer: Here's a 1796 tome with a brazenly immodest title, "The Immortal Mentor: or, Man's Unerring Guide to a Healthy, Wealthy & Happy Life." Promotional blurbs? One of the first in the nation appears on the cover of "The Immortal Mentor"--from no less than George Washington. "I have perused it with singular satisfaction," the general avers, and found it "an invaluable companion."

Textbooks designed to appeal to children? Here's the vastly successful McGuffey's Reader series, which sold at 12 1/2 cents in 1844 and eventually ran to 125 million copies. There are cheerful drawings facing the title page, with enticing captions like: "Albert and his Dog.--See page 68."

Romance magazines? A copy of The Saturday Evening Post for Aug. 27, 1898, carries this caption for one luscious illustration: "Wilmot held his breath when she began to sing." Novels in magazine format? Here's Jane Austen's "Mansfield Park"--in 1881, it cost 20 cents, "unchanged and unabridged." Mass-market paperbacks? These Civil War-era copies of Beadle's Dime Novels ("the choicest works of the most popular authors," including "Kent, the Ranger") have page edges worn to the texture of pussy willows by eager thumbs a century ago.

The show begins with the so-called "Indian Bible" of 1663; translated into Algonquin, it was the first Bible printed in the colonies. But the fun starts with the popular reading from the Revolutionary period. A 1792 chapbook titled "A Wonderful Discovery of an Old Hermit, Who Lived Upwards of Two Hundred Years," may defy credulity, but is "confirmed by them who were eye-witneffes." Equally astonishing are the school books, including the 1790 "New England Primer" with vocabulary drills ("abominable, fornication") which would stagger a modern student and pronunciations ("generation" is given five syllables) very different from our own.

Nineteenth-century curiosities include the miniature copy of the Emancipation Proclamation, smaller than a Metro fare card, which northern abolitionists distributed by the millions; several editions of "Uncle Tom's Cabin" (3 million sold in the United States); and the popularized rip-offs of Charles Dickens and Sir Walter Scott which were common before international copyright laws.

By the early 20th century, the masses are reading "The Wild Women of Broadway" and thousands of similar titles from the Little Blue Book paperback series, which at 5 cents each sold some 300 million copies before its demise in the '50s. But there were limits to wildness, as seen in the copy of Henry Miller's ribald "Black Spring," published in Paris in 1936. A hopeful Miller sent it to Alfred Knopf. Knopf gave it to his attorney, whose warnings appear in the margins. Several ripe locutions are crossed out, and the lawyer has circled the words "drunken micks," commenting that "the church does not like these expressions--and the judge is apt to be a Catholic." Miller refused to endorse the changes.

Deeper into the century, we encounter the sturdy Armed Services Editions of Fitzgerald and Thurber distributed during World War II, specimens of The Big Little Books, including a novelization of Republic Pictures' "Gene Autrey in 'Public Cowboy No. 1,' " as well as the Little Golden Books, and a sheaf of Classic Comics and their slightly higher-brow cousins, Classics Illustrated. Who can forget the 1952 "Hamlet," with a voluptuous Ophelia in an off-the-shoulder robe and a rakish, bare-chested Hamlet whose entire 35-line "To be or not to be" soliloquy is jammed into a single word-balloon?

Twelve years of changing paperback covers for Joseph Heller's "Catch-22" are here. So is Truman Capote's manuscript of "In Cold Blood" (a green-tinted spiral notebook with pencil writing as tiny and crabbed as Capote's voice). And by the time the show ends--with the reader's report that caused the Book-of-the-Month Club to reject Raymond Chandler's "The Big Sleep" ("This bird has some of what Hammett had," but "the range is pitifully limited")--the viewer is very sorry to see it go. "A Nation of Readers," sponsored by a grant from the Book-of-the-Month Club, will continue through September 9.