PAPERBACKS are books disguised as magazines. Despite 19th-century Tauchnitz editions and Beadle's Dime Novels, the "paperback revolution" only took place after Pocket Books began to be distributed by magazine wholesalers to cigar stores, train stations, and five and tens. Any book that didn't sell there could be stripped of its cover, the cover returned for full credit, and the remaining pages destroyed.

Although horrifying to readers, this practice points up the paramount importance of the paperback cover. Hardbacks sell by author recognition, word-of-mouth, reviews; not so the paperback: The cover sells the content, since much paperback buying is thought to depend on impulse. Packaging must therefore attract the passerby, encourage him to pick up the book, read its endorsements, look over the opening pages. And, if all goes according to plan, buy it.

Such a process, recalling the street pitch of the prostitute, largely accounts for the sexual content of paperback art: the book becomes erotically charged, reading ceases to be study and becomes . . . a real pleasure. Early covers depict brazen molls, sloe-eyed nymphets, and sullen backwoods slatterns offering themselves, ostensibly to some dark masculine figure in the background, but in fact to the reader. Later, post-women's liberation, the sexual appeals become more ambiguous: in Blue Skies, No Candy an elegant woman's hand, her fingernails like drops of blood, tugs at the zipper of a man's tight denim jeans. Whose fantasy is this?

The intimate history of paperbacks may be read in such covers, many reproduced in this lavish Penguin paperback. Next to the provocative art, however, Thomas Bonn's text displays a dry scholasticism, thus missing the excitement of his subject though providing much useful information. For example, Phyllis Whitney's Thunder Heights is generally considered the first modern romantic gothic, its cover picturing the once-standard elements of woman in white fleeing from dark castle, a solitary light shining from the bell tower. Gold Medal Books started the paperback original back in the late '40s, one of their chief writers being the prolific John D. MacDonald. J.D. Salinger so hated the Signet cover for The Catcher in the Rye (Holden in red muffler and cap) that he switched to Bantam Books.

Much of what Bonn says will interest the collector or student of publishing; but the cover paintings here by James Avati, Robert Jonas and many others provide the really fascinating text in the history of paperbacks.