XNA Shader Programming – Ambient
http://digitalerr0r.wordpress.com/2009/03/23/xna-shader-programming-tutorial-1-ambient-light/
XNA Shader Programming – Ambient and Diffuse
http://digitalerr0r.wordpress.com/2009/03/23/xna-shader-programming-tutorial-2-diffuse-light/
XNA Shader Programming – Ambient and Diffuse and Specular
http://digitalerr0r.wordpress.com/2009/03/23/xna-shader-programming-tutorial-3-specular-light/