I’m really impressed by your open sharing of your experiences in authoring an open text book. Nice one!
A minor point: Have a think about a consistent and scalable way of managing the attributions and licenses of the images (and other rich media) you plan to incorporate into your open textbook to facilitate downstream reuse and packaging. Check how the epub and html outputs deal with theses attributions when exporting your text.
An ideal solution is one where you simply link to a url which hosts the image and corresponding metadata — and the open textbook software automagicaly handles the attributions of openly licensed objects using CC’s machine readable code. From memory, I don’t think PressBooks has that capability yet (although this may have improved since I last looked.)
You’ll save a ton of time if you need to change how you are attributing images etc by sorting out a solution before you start authoring.