I have a book about my C64 which has a <i>commented</i> ROM listing in the appendix. This was basically the first professional code I read.<p>The book has a fold out plan of the schematics and there is one chapter explaining every part of it and one chapter about C64 repair. This is were I learned a good deal of electronics.
There is no such things for the Pi. OK the complexity is higher but for me, exposing GPIO interface doesn't just make the platform an open platform and though, C64 was not more open.<p>It's hard to find good doc of that level for modern product which plays the game of the openness. Well, I didn't check yet the Novena project.