We’re slowly building our Guide to PressBooks.
It’s a big and too-slow project (and, if you’d like to help, let us know!), but coming along. In general we’re adding new chapter/how-tos as we get help requests, and as we add or tweak new features. We’ve just added a new chapter to the guide: “Chapter Author and Subtitle (Works for Front-Matter and Back-Matter as Well).”
This tells you how to add chapter subtitles and authors, and what happens when you do.