Pathway permitted a number of different outputs, but it is apparently not being maintained and is currently not available from within Paratext. Can PTXPrint take up the challenge of exporting to eBook format? Or can Pathway be revived?
An ebook is basically a set of webpages wrapped up in a zip file, where the layout is controlled by the ebook reader and the page can be reflowed, etc.
PTXprint is a python front-end to do some pre-processing on the USFM and otherwise to generate the configuration files for XeTeX, which then generates PDF files, controlling the positioning of each letter on the page, footnotes, etc.
While there are some ways of generating HTML from a small subset of certain flavours of TeX input files (e.g. LaTeX) I’d be very very surprised if they reliably produced usable results from the ptx2pdf macros, it would be better to go directly from the USFM to HTML.
I don’t know, but I wonder if scripture app builder has the capability. It would be a much much smaller leap to epub for SAB (assuming it doesn’t currently do it) than for PTXprint.