1.How relevant it is for GLSL developers ? [/quote]
It is as relevant as you want make it to be. If you use OpenGL as the primary API for your application, then GLSL is absolutely relevant. If you opt for Direct3D, then obviously HLSL is more relevant.

No matter which API you opt for, there is no reason why you can't study shaders from the other camp. Looking at the CUDA and HLSL stuff is still useful even if you develop for GLSL. Conceptually they are similar. More importantly, when you read articles in GPU Gems, understanding the algorithms and the techniques is the key here, which can be re-implemented in whatever language you desire.

2.How relevant it is for GLSL on ATI Radeon card ?[/quote]
ATI has good support for GLSL and so does nVidia. I don't think you will need to worry much, apart from little quirks with standards compliance (nVidia is known implement some non-standard GLSL features, for example).

I don't know any 1 good book on openGL shaders. There are a couple online places like ozone3D, developer.nvidia ( I suggest you read their slides as they are not any language but usually theory and graphs, they also have a SDK 10 and 9.5 to download). GPU Gems I think all 3 are free online now, but their source code is usually DX, but there is a lot of theory and ideas to understand what and why.

Books like GPU Gems, GPU Pro or the ShaderX series are about techniques, algorithms and math. A 3D API is just a tool to implement these algorithms and is completely interchangeable. In practice both HLSL and GLSL is used by articles in these books for their example implementations. Many are completely API agnostic and entirely math centric (with maybe some shader pseudo-code).

Just a note of warning though. Especially the later books in the series contain some very advanced research material that assumes a very firm understanding of your target shader API. These books will absolutely not teach you how to write shaders.