Hmmmm... this is a good question. I have a hardback book called "The Crochet Book Bible" that I reference a lot. An online reference that I really like too is http://www.stitchguide.com/stitches/crochet/
I use so many, I really don't have a single favorite. I did buy a book recently called The Cool Girls Guide to Knitting. It has some cute patterns and since I never took a class or anything, I could use it to brush up on techniques.
Hey JJ, thanks for the link. I added it to my favorites. Great page!
I like to use that website also. And the best reference is... right here! All questions answered, all the time. :)