Mobile 3-D Graphics: with OpenGL ES and M3G

Authors: Kari Pulli, Tomi Aarnio, Ville Miettinen, Kimmo Roimela, and Jani Vaarala 

http://www.amazon.com/Mobile-3D-Graphics-Kaufmann-Computer/dp/0123737273/ref=sr_1_1?ie=UTF8&s=books&qid=1262658452&sr=8-1

This is a very thorough text on the kind of 3-D graphics used on the iPhone.  This book doesn't just do a good job of covering OpenGL ES, it also provides a very thorough introduction to the principles of 3-D graphics in general.  The book was published before the iPhone SDK came out, so no mention is made of that device in particular, but the information contained within is just as relevant.  This is my go-to book for when I need to dig a little deeper into OpenGL ES or do a little performance optimization.