I'll wager J.R.R. Tolkien's 'ring' trilogy makes it on more people's 'gazillion' list than just about any other books, but I wonder why that is? Yes, yes, they're well-written in many ways, but that alone hardly explains their apparent broad-spectrum appeal. But there are many many other well-written books out there meeting that criteria and alot of the same folks somehow don't manage to read them a gazillion times.
To clarify, by 'gazillion' we really mean books you have systematically read cover to cover three or more times in your lifetime.
It's pretty rare for me to enjoy a book so much that I feel it warrants three or more reads, and truth be known I wouldn't say that those books are necessarily the greatest literature either. If anything they tend to be escapist in nature and most 'fun'. I've read Patrick Obrian's Aubrey/Maturin series (20 books in all) four times now. I've also read the original 'Hitchhiker's Guide' three times, James Clavell's 'Shogun', and Alexandre Dumas' 'Three Musketeers' thrice as well.
What books have you read a gazillion times?
Oh, and for the record I'm guessing I've read the ring trilogy maybe 8 times since junior high.