You're stuck inside Fahrenheit 451. Which book do you want to be?
A little secret here... I hate Bradbury. But... if this really is just a poorly worded way of asking: what fictional universe would you most like to inhabit? The answer is indeed simple; Forgotten Realms.
*** UPDATE ***
Its occured to me that this question is actually asking, if you were in 451, what one book would you memorize? None. No way I'd be spending time trying to save books by memorization. I'd be spending my time loading ammo, and killing firemen. Violence is the answer.
Have you ever had a crush on a fictional character? No. You freak.
The last book you bought is? War Songs and Poems of the Southern Confederacy by An Ex-Confederate
What are you currently reading? Recently finished The Ballad of the Whiskey Robber. Non-fiction. Very good... sort of a Truth-is-stranger-than-fiction deal.
Five books you would take to a desert island: Watership Down by Richard Adams, My bound copy of the complete, and increasingly inaccurately named Hitchhiker's Trilogy, The Sound and The Fury by William Faulkner, Zen and the Art of Motorcycle Maintenence by Robert Pirsig, and of course... The Bible.