The current ePub isn't put together right, so it just has headings for the pages and no actual content. You can find the docs in the archive as Markdown files.
I've identified the issue. A correctly built copy of the ePub is at https://forthworks.com/retro/r/RETRO12-2021.1.epub (replacing the broken one) and I've now issued a 2021.2 release to fix the issue that caused this.