A pair of books licensed in the Creative Commons (BY) which talk about the architecture of open source software. The core developers of four dozen projects talk about how their software is structured, and most importantly why. They were written so that F/OSS developers wouldn't have to learn by reinventing the wheel, instead there would be a reference.
There are two books, which you can read online and download for free. Or you can buy them as Kindle editions (the proceeds go to Amnesty International).