Amusing guide to B-movies; The Psychotronic Encyclopedia of Film, by Michael Weldon. New York: Ballantine Books. 802 pp. $16.95 (paperback).

According to the press release, if you've ever seen a film in a theater that's since been turned into a parking lot, then you've probably seen a ''psychotronic'' film. This handy volume provides information and amusingly honest reviews of over 3,000 science fiction, horror, fantasy, exploitation, and many other movies that are considered out of the mainstream. It also covers higher-status films graced by the appearances of such horror greats as Vincent Price and Boris Karloff.

Michael Weldon previously edited a weekly New York guide to the best of the worst movies on local television, which he dubbed ''Psychotronic.'' Now, with some assistance, he has produced this compilation. While there are many good film reference works, few are devoted to such gems as ''Abbott and Costello Go to Mars'' (in which they go to Venus), ''Hellcats of the Navy'' (starring Ronald Reagan and Nancy Davis, before they became President and First Lady), ''I Was a Teenage Werewolf'' (with young Michael Landon), and the Japanese classic ''Destroy All Monsters'' (featuring Godzilla and an all-star monster cast).

Due to the violence and sexual themes of more recent horror entries, some discriminating film buffs may be turned off by a few of the several hundred stills and ads that illustrate the volume. However, for the cinematic sociologists who feel they can learn as much about a culture from its mass-market entertainment as from its most serious works of art, this book is an essential reference work.