You can learn more about PreTeXt from some of these presentations and papers. Of course, the newer ones are more accurate.

PreTeXt: An XML vocabulary for scholarly documents, Robert A. Beezer. Presented at Balisage: The Markup Conference 2018, Washington, DC, July 31 - August 3, 2018. In Proceedings of Balisage: The Markup Conference 2018. Balisage Series on Markup Technologies, vol. 21 (2018). https://doi.org/10.4242/BalisageVol21.Beezer01. [Paper] [EPUB]

Sound Writing: Reclaiming Authorship in the Digital Age, Julie Nelson Christoph, Kylie Young, Kieran O’Neil & Cody Chun. JoSch - Journal der Schreibberatung 01/2018. (Reprint available after a one-year embargo.)

A New Generation of Mathematics Textbook Research and Development, K.L. O’Halloran, R.A. Beezer, D.W. Farmer.
Special Issue: Recent Advances in Mathematics Textbook Research and Development.
ZDM Mathematics Education, Gert Schubring, Lianghuo Fan (eds.). (2018) (to appear, now online)
https://doi.org/10.1007/s11858-018-0959-8

MathBook XML, Robert A. Beezer
TUGboat: The Communications of the TeX Users Group, 35, no. 2 (2014), 209-211.

Sage-Enabled Textbooks, Robert Beezer. Sage Days 88: Opening Workshop for a Year of Coding Sprints, Institute for Mathematics and Its Applications, University of Minnesota, August 2017.

Authoring Online Open Textbooks with MathBook XML, Robert Beezer, David Farmer. Second International Conference on Mathematics Textbook Research and Development, Rio de Janiero, Brazil, May 2017.

Open Software and Open Textbooks for Mathematics, Robert Beezer. Korean Mathematical Society Invited Lecture, Annual Spring Meeting of the Korean Mathematical Society, Chosun University, Gwangju, Korea, April 2017.

Textbooks for the Web from MathBook XML, Robert Beezer. Presentation from the 2017 US Joint Mathematics Meetings as the WEBSIGMAA Guest Lecture. A good introduction to open textbooks and PreTeXt. Includes more technical details than usual, with a good example of source XML converted to LaTeX and HTML. To mimic the demonstration portion of the talk, explore the online version of Tom Judson’s Abstract Algebra: Theory and Applications.

WeBWorK-MathBook XML Bridge, Robert Beezer, Alex Jordan. A project from Summer 2015 to incorporate WeBWork problems into PreTeXt documents, supported by OpenOregon.org and the University of Puget Sound. Report of the project’s work in a streaming webinar, along with plenty of technical hiccups, especially a gap from about 34 minutes to 43 minutes. December 2015.

Mathematics Department Colloquium, Robert Beezer. University of Manitoba, Manitoba, Canada. May 2015.

Overview talk at Sage Edu Days 6: Part 1  Part 2, Robert Beezer. June 2014.

History talk at University of Puget Sound Math/CS Department Seminar: Modern Textbooks for Modern Times, Robert Beezer. September 2014.