We just posted a blog post about the re-write of Jupyter Book!
This is a total rewrite of the codebase to be built on Sphinx and 
in Python
Before I forget, let me say this looks amazing from multiple angles:
I mentioned it on Twitter and have gotten a lot of enthusiasm in chemistry / physics.
I just started playing with jupyter-book this weekend. 
This is exciting.
As a notebook matures I often hoist python into modules. Is there a way to include just plain python modules in a book?
I found this link about Jupytext I want to vote for supporting python source files. I guess I expected that I would find that as an issue, already.
See this section for instructions on how to get .py files included in your book: Custom notebook formats and Jupytext