If I remember correctly, the book was actually written after the movie was shot.
I always thought the book came first, but it didn’t.
Interesting article about it here at wikipedia.
2001: A Space Odyssey (film)
http://en.wikipedia.org/wiki/2001:_A_Space_Odyssey_%28film%29
The book and movie were actually done concurrently, with some script changes, but not all, included in the book.