Also, sometimes some interesting article appears here or there, like:
http://dave.cheney.net/2013/06/02/why-is-a-goroutines-stack-infinite. But I don't know of any attempt at a central index of those.
As to reading the source, there's actually more to this than what appears on surface, as in several places of the (C) code, there are big comment blocks, which contain a fairly high-level overview of various subsystems and interactions, in regular (technical) prose. Now that I think of it, it could be a good idea to write a script which extracts blocks of kinda >= 3-line comments, and dumps them together into one pseudo-book, to extract all of those gems and collect them in one place.
Also, I've once tried to seed a small wiki with some info I found about the internals, at least on some things which were highly non-obvious to me when trying to start the dive into the code - see:
http://goin.wikispot.org/. One guy once added some more to that, but other than that, the wiki didn't see much adoption for now.
/Mateusz Czapliński.