Sean is right, his book is an excellent big-picture overview of Pidgin, regardless of its name. While he correctly barbs me for having not read the entire thing and provided comments (as I long ago told him, in a past life, that I would do), I have read enough of it to understand that it is in fact a fantastic resource for Pidgin developers, or indeed Open Source application developers in general.
That said, there was no content for big-picture documentation within the Pidgin documentation tree, as many people have pointed out. Consider my diagram (and any future efforts) a contribution to that particular anemic part of the Pidgin documentation. We would have to generate hundreds of pages of great documentation to eclipse the book, but we don't have to eclipse it to provide at least some useful content.