I want to know more about GPUs? Where to get information?

CPUs are fairly well documented, you can read about the different architectures (x86, ARM, etc), find simplified implementation, read sample micro-code, etc. I wish to find that type of information about GPUs, what is special about the framebuffer, when is information going from main memory to GPU memory, when this GPU memory is accessed, what is read from it, under-which format things are stored in GPU memory, etc.