Well, just focusing on single books and leaving aside series like LoTR, Harry Potter, Wheel of Time, etc, my first thought for longest book was the uncut version of The Stand by Stephen King. However, this page here shows that the unabridged Count of Monte Cristo is nearly the same length, at 464,234 words versus 464,218 for The Stand, though as I've read an English translation of Monte Cristo, word lengths may vary somewhat by translation. So...I guess I have two!
As for shortest, that might be some children's picture book, but what springs to mind is the joke book Everything Men Know about Women, which aside from the cover and some excited blurbs on the back, has over one hundred blank pages inside. So I guess that's the shortest 'book' I've ever 'read.' =)