When it comes to being informative, I love Vogue Knitting, the Ultimate Knitting Book. I consider myself to be an experienced knitter, and I STILL refer to it from time to time. It's full of techniques and info, not patterns.
For all those little tips and tricks knitters learn through experience, I love Elizabeth Zimmermann's Knitting Without Tears.
For stories about knitting and yarn, Knit Lit and Knit Lit Too. Just good reading when you can't knit.
For patterns? Heck, I haven't met a pattern book I didn't like in some way, and I'd be hard pressed to choose a favorite.