The documentation is "complete" in the way that some manpages are - the biggest problem with it is the lack of best practices guidance. About books, I believe that good ones exist now. The official CMake book is not among the good ones - last time I checked, it was ridiculously outdated (and IMO not even very good at the time it was written).

I have extracted some knowledge from the source code. It's more underdesigned than overdesigned and I think that's the better side to err on. Kinda awkward but you can figure it out.