I noticed that this series of free books from Nvidia is really popular and well know in the devs world, the problem is that is simply old; you would recommend to read those 3 books to a developer that wants to improve his skills with a programmable pipeline and is starting to do so in the 2012 ?
The articles in the GPU Gems are more about techniques and not really about the technical abilities of the GPU offered through a revision of one graphics library or another. Some of those gems are really gems: explaining a concept that is still of interest, both in the means of approaching the problem and in implementation details. Before this gems series, I think there was another one focused on assembly language shader code. Much of those techniques were converted to a more readable higher shader language form and are still popular if not central today.
Technically, the GPU gems offer ideas for developers using HLSL up to 3.0 (DirectX 9.c) (the code is actually in Cg, a good dialect of HLSL). Equivalently, in OpenGL 3+, most core techniques are programmed the same way, using the same ideas and concepts. So, if you're not into getting the latest techie implementations running and want to get a good solid knowledge base, they're still a nice reference.
If not restricted to OpenGL, a newer series is the follow-up of ShaderX: the GPU Pro series (if you tolerate the DirectX approach and can translate the techniques to OpenGL).