I guess most manuals are written after-the-fact; documenting a program that's already there. This is not how this manual is written. When implementing something, I write the manual entry for that something straight away. I then see that it's difficult to explain the functionality, so I write how it's supposed to be, and then I change the implementation. Writing the documentation and writing the code go hand in hand.
This, of course, means that this manual has no, or little, flow. It documents absolutely everything in Gnus, but often not where you're looking for it. It is a reference manual, and not a guide to how to get started with Gnus.
That would be a totally different book, that should be written using the reference manual as source material. It would look quite different.