[bokumo]

You should take a look at PreTeXt. https://pretextbook.org/

[capnrefsmmat]

I actually was just looking at it a few days ago. Interesting project, and looks designed exactly to produce academic books and texts. But it's not programmable in the TeX sense, where you can extend it to do new things within the language -- it's just that PreTeXt's base features include most of the things I want.

I'm trying to resist writing another book, since I know how much time it would suck up, but if I do, I'll probably look at PreTeXt and maybe Scribble as options.