Antiquarian book fairs are more common than some people may imagine. Most major cities are sure to have one, but events are scheduled in many smaller communities as well, from Bangor, Maine, to Glendale, Calif. Following is an abbreviated schedule of future book fairs. For additional information, contact the Antiquarian Booksellers Association of America Inc., at 50 Rockefeller Plaza, New York, NY 10020, or call (212)-757-9395. April 21-22, Cambridge, Mass. April 23, Ann Arbor, Mich. April 28-29, Louisville, Ky. April 29-30, New York May 5-6, Lancaster, Pa. May 7, Berkeley, Calif. May 12-14, Toronto May 13, Timonium, Md. May 20, Albany, N.Y. May 25-27, Chicago June 4, Concord, N.H. July 16, Monterey, Calif. July 22, Sandwich, Mass.