Jinxed perhaps by the old bromide that you can't judge a book by its cover and by row after row of books shelved spine out in the stores, book-jacket art has become the unglamorous and unglorified stepsister of the world of commercial art. But everyone who reads has undoubtedly come across the cover art of Wendell Minor. For 25 years, his distinctive paintings, with their paradoxically mysterious clarity have graced the books of a wide range of writers. The grim Hopper-esque realism he produced for Mary Higgins Clark's Where Are the Children, the fusion of reality and fantasy for W.P. Kinsella's Shoeless Joe--both illustrate perfectly his philosophy: ""A good cover has to have a sense of time and place, a sense of the atmosphere of the book . . . I think of it as a picture puzzle with one or two pieces missing. Only by reading the book will they be found."" This celebration of his art includes scores of Minor's book covers as well as an appreciation by David MuCullough and interviews by Minor with his mentor, Paul Bacon, and Simon & Schuster's longtime art director, Frank Metz.