Some used to write books using LaTeX a quarter of a century ago and it was pretty much a build system. You'd typically have a little shell script making backups, cleaning the mess from the last build, building the book in x passes (IIRC there was a first pass for the book and a second one for all the references), converting it to another format, maybe opening the result in a viewer, etc.

As for TFA: 640 pages in 15 months ain't bad. Some have a lot of discipline and can write five or ten pages a day, but that's really hard...

Check out leanpub.com ... basically what it is.