Here's the order in which I've read them (or at least what my scrambled mind can remember).

Skinny Bitch by Rory Freedman and Kim Barnouin  (this book changed my life!)
I'll also mention Skinny Bastard which Brian loved.  It changed his view of food.

Eat to Live by Joel Fuhrman, M.D.  (if you actually follow his advice the weight falls off)

The China Study by Dr. T Colin Campbell  (long read, but life changing)

The Kind Diet by Alicia Silverstone  (more about macrobiotics, but some good info)

The Veganist by Kathy Freston  (this is a great introductory book to Vegetarianism)

Crazy Sexy Diet by Kris Carr  (this should be read before China Study, etc.  Good book)

Unlimited by Jillian Michael   (This is all about overcoming mental obsticles.  Freaking amazing book!)

The Spectrum by Dean Ornish, M.D.  (A good common sense approach to eating.  Don't worry, not a vegan or vegetarian book).