MOST OF US are taught in childhood never to write in book margins, but people do anyway, and sometimes the rest of us are delighted.

In fact the best part of the new rare book exhibit at the Library of Congress is the stuff written in the margins.

These are 32 books, pamphlets and broadsides collected from 1800 to 1926; the exhibit itself represents the dropping of the other shoe after the library celebrated 50 years of the rare book reading room in 1977. This show lasts through the year, and it is a fine way to spend a quiet lunch hour, especially if you have a magnifying glass.

Quite likely the most interesting piece is Ben Franklin's copy of a 1769 booklet on the nature of government relating to Britain's disputes with her American colonies.

In a fairly legible hand, Franklin carries on a running battle with the author, attacking his sources and his claims. One wishes more than two pages could be exposed, a standard problem in exhibiting books. Considering the date, it is fascinating to get in touch with Franklin's thoughts on government, even in this fragmentary form.

On a yellowed flyleaf: "John Adams to Thomas Jefferson, Jan. 1, 1812." It is an old man's writing, wobbly and trailing but still clear and concise. Even clearer is Jefferson's tiny set of notes on a copy of "The Federalist," the rounded, neat letters as readable as type. There are other autographs, from Mark Twain to Harry Houdini, and a mystery: A note scribbled in the corner of a 17th-century broadside may upset historians who believe that printing in Boston dates from 1675, because this is dated 10 years earlier.

One remarkable short book, regarded as the first printed work of an American black, is "A Narrative of the Uncommon Sufferings and Surprizing Deliverance of Briton Hammon, a Negro Man," dated 1760. One of two copies known to exist, the book follows Hammon from the time he left his master in 1747 only to be cast ashore in "the capes of Florida," tortured and imprisoned for nearly five years by Indians and Spaniards. The story, the first of the American slave narratives, ends with a hymn of thanksgiving for his rescue. The exhibit also features an account of a 1741 uprising of slaves in New York City.

Curiosities in this modest show range from the 12-cent first edition of Poe's "Murders in the Rue Morgue," which sold eight years ago for $20,000, to an "Alice in Wonderland" with two original Tenniel pencil drawings bound in and a first edition of Izaak Walton's "The Compleat Angler" featuring an antique misspelling. On an inside page the title is given as "The Complete Angler." Maybe the printer knew something.

But there is more to see than curiosities. There are incunabula, some beautifully illuminated old volumes and the kind of clean, sharp printing, all but engraved into the paper, that once led an expert to say, "Printing started with perfection, in Gutenberg, and we've been trying to work our way back to it ever since."