WITH BOOK COSTS up there next to caviar, hard-core readers are learning to band together into book clubs - before they go broke. But some people have been doing it for years. The oldest and sturdiest local group is the Cleveland Park Book Club ("the only exclusive club in Cleveland Park"), which has been in business for 46 years. It was founded by a woman who moved here from a small town in Vermont, where the library couldn't afford to order current books, so a group of neighbors bought them, shared the costs, and passed them around among themselves.

Only 32 families can belong to the club, which operates for 32 weeks between October and May. The current president is Mrs. Peter Craig, and members include Mr. and Mrs. James Rowe and Judge Gerard Reilly, former judge of the D.C. Court of Appeals. Dues are $8.50 per family a year, and each fall the club's selection committee chooses 32 fiction books and 32 nonfiction, based on lists suggested by members and a rigorous study of all the book reviews in sight The books are circulated in pairs - one fiction, one nonfiction - and the deadlines are the stiffest in town. Every Monday, members must pass their two books to the next family in line, whether they're ready to or not, and latecomers are "given a polite nudge." Some hollow-eyed readers barely deliver the goods at midnight of the appointed day, and a few hardy souls have survived the pace for 20 years or more.

At the end of the season, the club holds an aution, where members bid on their favorite books (and pay a fine if they don't attend), and the money raised helps buy next year's books and keeps the dues low. If members leave town, their place is swiftly taken by the next family on the long list of readers wating to join.

Another group, also called the Cleaveland Park Book Club, is in the middle of its first year. Organized by Mrs. William McDaniels, the 23 members - including newsman Alfred Friendly Jr. and attorney Teery Lenzner - read 46 books. Members are allowed two weeks before they pass their two books along - until March 6th, when they speed up to a one-week deadline until their season ends May 28th. There is also a group in Georgetown - led by Joya Cox, an alumna of Cleveland Park, and Virginia Adams, the wife of the rector - in which 16 resolute readers from St. Mark's Episcopal Church on Capitol Hill pass their two books along every other Sunday - at church.