Although my childhood movie-viewing habits were comprised entirely of science fiction/fantasy/horror and historical adventure films (Ray Harryhausen, Willis O'Brien, Boris Karloff, Charlton Heston, Michael Curtiz, Franklin Schaffner, etc.) and certainly indicated a strong affinity for imaginative fiction, the book that permanently warped my consciousness with shattering impact was The Hobbit by J.R.R. Tolkien. This gracious accident of Fate would later have serious repercussions on my future and colour my decision to foresake a career in science for that of a commercial artist.
I read it when I was 11 and immediately launched into the Rings Trilogy, thus cementing my postion in my hometown as the resident, certifiable Tolkien fanatic (why the DSM IV doesn't categorize this as a variety of obsessive-compulsive disorder is beyond my understanding).
Next, I read Ray Bradbury's The Martian Chronicles and Arthur C. Clarke's Childhood's End by the time I was 13. Reeling from that devastating one-two knockout punch and late in my 14th year I read H.P. Lovecraft's The Shadow Over Innsmouth which sent me spinning over the edge, sealing my fate forever as a purveyor of the weird and wondrous.