This is the work of Hugh McGuire. It's based on WordPress,which is good for me because I have so much experience with it. But this is also good for the novice because WordPress is at its core a publication tool. The original blog design took the text and images entered by an author and then displayed them automagically on a webpage.
The key detail here is that converting and displaying the content is automatic. If you write the correct scripts you can have the WordPress back end do just about anything - including make ebooks. And that's what Pressbooks has done.
Their site is running a heavily modified version of WordPress which will let you add content by chapter or section. You can also define various parts of the front matter, including the copyright page preface, and more. Organizing and rearranging the content was easy, and a good part of the busywork (including things like the TOC) is automated.
Right now you can export Epub, PDF, and a raw form of XML. The PDF is intended to be print ready file for POD systems. I suppose you could read it on your computer but that's not the goal. The Epub output is specs compliant, and it supports embedded audio and video (but little else is supported from Epub3).
Full Epub3 support is coming, and they are looking at offering Kindle output.
Pressbooks is free for small publishers and the self-pubbed, but if you want to process dozens of titles at a time you will need to negotiate license.