NVIDIA released 2 great books (free) very recently which are a must for anyone who tinkers with graphics and specifically with shaders. First one is the GPU Gems book, which I happen to have as a h/c (, long before it was released). It’s such an invaluable resource of tricks that are still very much valid to this day. I would recommend this to anyone and everyone who wants to get their hands dirty with graphics. Then yesterday  The Cg tutorial was released. I haven’t got a h/c version of the book as I pretty much have a good hand at HLSL and both (HLSL and Cg) are essentially the same. I read thorugh the book and was equally impressed with it. So if you haven’t already read them, I would strongly recommend taking a good hard look at both.

EDIT [13th May, 2008]: Another one released today. GPU Gems 2.