"In the last five years this business has gone crazy. Everyone's specializing. Everyone knows the market. You don't get surprised anymore. Well, no, I won't say that. There are always surprises."

Andre Barbeau is a part-time dealer in old books on travel, and yesterday he came in from Falls Church to roam the 50 stalls at the Washington Antiquarian Book Fair in the Rosslyn Ramada. So far, no surprises, but he had all day ahead of him (and today from 10 a.m. to 5 p.m.). The fair benefits Concord Hill School of Chevy Chase.

The dealers came from as far away as New Hampshire. They go in for curious names like the Family Album, The Haunting Book Shop, The Printer's Devil and Wayward Books, and they were offering just about anything you could put on paper: first editions, incunabula (books printed before 1501), autographs, Life magazines, old prints and maps, drawings, manuscripts, engravings, antique college diplomas, Victorian tintype albums, land a book on "The Art of Handbell Ringing."

One surprise: A coffee-table book of Andrew Wyeth's paintings which cost $75 when it came out in 1968 already is up to $500, torn jacket and all.

"My husband is bringing a Gutenberg Bible page down tomorrow," said Mrs. David L. O'Neal of Peterborough, N.H. "It'll go for $5,000. It's our top item." (If you wonder what a complete Gutenberg goes for, Hans Kraus, a New York bookdealer, found out this week. He sold one to the Gutenberg Museum in Mainz, German, for $1.8 million, exactly what he paid for it in 1970.)

The O'Neals also have seven incunabula ($700 to $2,500) and a 1697 Cotton Mather.

Also, a charming item by Mather's father, Increase, titled, "The Answer of Several Ministers in and Near Boston, To What Case of Conscience, Whether It Is Lawful for a Man to Marry His Wives Own Sister?" This one is dated 1695 and goes for $2,200.

Of course, not all the items are all that ancient. There is a document written by Charles A. Lindberg authorizing Dr. John F. (Jafsie) Condon to negotiate with the Lindbergh kidnappers. There are signatures of presidents. There is a "Finnegan's Wake" signed by James Joyce from a limited edition of 425 copies ($1,000) and any number of modern first editions!

You can also find an occasional example of fort-edge painting, the first Wiggle Pictures, in which a painting materializes on the page edges of the closed book when you bevel it a little. These hidden pictures were a great favorite with pornographers, naturally. But such items are rare.The Vatican bicked them all up centuries ago.

Most of the business yesterday was being done by the dealers themselves, with a sprinkling of librarians, casual collectors and general book enthusiasts. The competition, dealers say, is terrific.

"We get calls from 7:30 in the morning to 11:30 at night," remarked Mrs. O'Neal. "These collectors get so frantic. They'll do anything to make sure their rival doesn't get some item."

She sends out six mailings a year to 3,000 customers, does a solid drop-in business in the Peterborough area, where many well-to-do Bostonians summer.

How does a person get into the business?

Well, it comes on gradually, in most cases. David O'Neal was a writer who turned to bookselling for a living 15 years ago and opened his own shop a decade later. At the moment he has about 5,000 items, of which he brought 300 to the fair.

He and his wife find them at autions, in private library sales, at university duplicate sales. For specialists particularly it is getting a bit cut-and-dried now, with values of many treasures firmly established. And an expert usually knows more or less what he's going to find in his competitor's shop.

But there's always that dusty volume in someone's attic, that dingy poster on a junk shop wall, that old letter stuck between the pages of an overlooked book.