One of the better ones in that space is https://www.softcover.io/

The build pipeline looks like:

  .md --> .tex  -->  .pdf/.html/.epub/.mobi/.html

or you can start directly from the .tex

  .tex  -->  .pdf/.html/.epub/.mobi/.html

If you're looking to write a book that has a lot of latex math equations in it, this is the "book builder" you need. The tech is open source, see https://github.com/softcover/softcover and https://github.com/softcover/polytexnic