PreTeXt
Write Once, Read Anywhere

These are other projects I have used or studied, which have similar aims or approaches. (Where dates are not given, descriptions are from around March 2014 or so.)

tbook
Very similar to PreTeXt, but pre-MathJax so working very hard on the math. Inactive.

NoteShare
Many formats from one source. HTML functionality is much like MathBook's. Source format is ASCIIdoc. (2014/12/15)

Text Encoding Initiative
More about exchange and preservation, less about authoring. More about humanities and less about science.

DocBook
Excellent choice for technical documentation, maybe not the right choice for academic work.

Pressbooks
WordPress as a front-end to PDF and EBook production. LaTeX support is limited to the WordPress “short code,” which will only create inline snippets as PNG images. (2016-10-19)

Softcover
“Frictionless self-publishing.” Authoring language is a unique blend of Markdown and a subset of LaTeX.

GitBook
“We are building the best tools to help authors publish and sell books.” Proprietary editor, Markdown syntax and “TeX support is planned.” (2014/11/16)

Ximera
“Structured LaTeX” as an authoring language. Allows for constructing interactive exercises with data analaysis on student answers.

TeX4ht
Best of the converters from LaTeX source. Very steep learning curve to customize or extend.

TTH
Another converter from LaTeX. Free to use, but copyrighted with non-standard (weak) license.

LaTeX2HTML
Another converter from LaTeX.

dblatex
Convert DocBook to LaTeX.

ReST: reStructuredText
Lightweight markup, process with Sphinx to various outputs.

Pandoc
A “universal” document converter.

AsciiDoc
“Text based document generation.” Lightweight markup similar to ReST or Markdown, convertible to multiple output formats.

Markdown
Lightweight markup convertible to HTML.

There are probably more. If you think something should be listed here, let me know.