Yes, using the Documentation/ as base to create a book is a good idea, but it should focus on "OS Components" and "API Reference".
I don't think that the documentation should be limited to simple HowTo and WhatIs documentation. I think it should have some technical meat too! Like explanations of how things work and why they work that way: Theory of Operation documents.
I really like Zephyr's Documentation, for example: https://docs.zephyrproject.org/latest/index.html . There is quite a bit of meat there; something for everyone.
