Only the development book is in an elaborated state, the other books are holding for more ready code.
And two of them are new:
And two of them are updates from existing documentation:
Together with the documentation we've introduced a wiki. Its purpose is to walk-through decisions, convention, terminology. It's where we document the reasoning the development team went through before implementing more sophisticated functionalities.
Finally, when the documentation doesn't explain, and to ensure the whole code is properly tested, we have the tests. And the coverage is available here. At the time of writing the coverage has 98.76% code lines tested.