Its hard for me to treat the books of this series separately (ShaderX 3, ShaderX 4, ShaderX 5). They are all very good books of GPU-Gems level or higher. In comparison with GPU Gems, they are more academic, i. e. they are rather short and more applicable to wide range of applications then GPU Gems ones (while GPU Gems series is more scientific, state of the art, considering one particular research) and the accompanying CD is much more better (lots of working examples, most with source code).

Sections (Image-Space, Shadows) are also very helpful to figure out what is useful for you.

This series is not for beginners anyway, so please, go Cg Tutorial or DX SDK Tutorial and don't put 2 or 3 marks for these books because you can't cope with them.