Nvidia has released a few days ago an online version of GPU Gems: Programming Techniques, Tips, and Tricks for Real-Time Graphics, the first tome of what I consider as one of the best GPU book series.
GPU Gems is a compilation of articles covering practical real-time graphics techniques arising from the research and practice of cutting-edge developers. It focuses on the programmable graphics pipeline available in today’s graphics processing units (GPUs) and highlights quick and dirty tricks used by leading developers, as well as fundamental, performance-conscious techniques for creating advanced visual effects. The contributors and editors, collectively, bring countless years of experience to enlighten and propel the reader into the fascinating world of programmable real-time graphics.
The book can be read here: http://developer.nvidia.com/gpugems.