Here are some links to a couple of quick guides and references that’s useful when writing shaders.

The OpenGL GLSL shader language Quick Reference Guide.
http://www.opengl.org/sdk/libs/OpenSceneGraph/glsl_quickref.pdf

The full specification of GLSL 1.20.8.
http://www.opengl.org/registry/doc/GLSLangSpec.Full.1.20.8.pdf

The full specification of GLSL 1.30.08
http://www.opengl.org/registry/doc/GLSLangSpec.Full.1.30.08.pdf

GPU Programming guide from Nvidia for both OpenGL and DirectX.
http://developer.download.nvidia.com/GPU_Programming_Guide/GPU_Programming_Guide.pdf

Guide to write shaders in DirectX9 in HLSL
http://msdn.microsoft.com/en-us/library/bb944006(VS.85).aspx

Guide to write shaders in DirectX10 in HLSL
http://msdn.microsoft.com/en-us/library/bb509703(VS.85).aspx