Hmm, the book is incomplete (the chapter about texturing is empty as are all subsequent chapters). Is there a blog following the development of the book? A twitter account? Anything?
I discovered these pages while looking for an OpenGL tutorial that actually used the new shader-driven style and not the deprecated fixed-function API. They're very good (although as others note, still a work in progress).
I've only glanced at it so far but if you're using ES 2.0 then it would be a big help as ES 2.0 has no fixed function pipeline so everything is done in shaders. The only real difference IIRC between OGL 3.0 and ES 2.0 is the unavailability of many extensions OGL users are familiar with.
The book appears to stay within the GLES2 subset of the API. It doesn't use the matrix stack, fixed attributes, stock lighting, nothing. It's all shaders. That's probably not an accident.
OpenCL looks like a generalization of the OpenGL shader language GLSL. The logistics of passing buffers and controlling execution should be very similar as well.
I think if you learn OpenGL 3.3 (as in this tutorial), you'll get enough of the common stuff down that you can fill in the missing spots from OpenCL reference material directly.
I'm only just getting into OpenCL (I already know OpenGL very well) and I'd say OpenCL's programming language is closer to C than to GLSL. In any case, I'd say the hard part is parallelising your algorithms, understanding the memory model (private vs local vs constant vs global pointers) and using it and the the task dispatch mechanism effectively.
Being experienced with OpenGL (or Direct3D, or game console) shaders will probably help for performance tweaking as the mindset is similar (memory access is evil, irregular memory access more so), but other than that I doubt it will help.
