DEAR MR. FANTASY by Ethan A. Russell (Houghton Mifflin: $29.95, illustrated). Russell was a restless American who went to London in the late '60s with ambitions of being a writer, but found his future in photography. Through a chance meeting with a journalist who needed someone to take photos of Mick Jagger, Russell parleyed what had been his hobby into a respected place in the rock world. His more than 300 photos in this handsome book are among the most prized of the era. They range from the Beatles' tense "Let It Be" sessions and the Rolling Stones' landmark 1969 and 1972 tours to such well-remembered album covers as "Who's Next" and "Hey Jude." There's an intimacy and warmth to Russell's best photos that conveys much of the innocence and corruption of the '60s rock generation. Far less successful is a lengthy text, which combines pieces of autobiography and social observation in an attempt to provide a wider portrait of that generation. The background on photo sessions is entertaining, but the sociological explorations lack authority.