“Programming Vertex, Geometry, and Pixel Shaders” by Wolfgang Engel, Jack Hoxley, Ralf Kornmann, Niko Suni and Jason Zink is a book published for free to the GameDev.net wiki and covers a wide range of shader programming techniques. It is covers mostly DX10 techniques and seems to be D3D-centric, but I’m sure there is plenty of knowledge to be gained even if you’re into OpenGL. It seems to be very thorough and even on a quick skimthrough, it seems more useful than a couple of actual physical books on shaders that I own.

The authors are working on a DX11 print-only version and are open to your comments and suggestions.

“Programming Vertex, Geometry, and Pixel Shaders” on GameDev.net Wiki