I checked out your product - nice! I think we have a similar approach for a different audience. EbookAloud is basically just a two-click conversion system more geared towards consumers than content creators. Upload an epub, get an audiobook. Users will have to live with mispronounced names, etc.

Content parsing and chapter alignment were also the better part of the work here. Laying out an epub on screen is straightforward. Extracting text by chapter without duplication or elision took a lot of iterations. I could not ultimately follow the epub spec recommendation to traverse the spine, but had to rely on the TOC to drive extraction or fallback to some simple heuristics if none was present. It’s still the biggest risk area in the code and why I added a detailed chapter breakdown before asking for payment. I’ve pushed a lot of content from a wide range of sources through the code for manual and semi-automated inspection and decided it’s good enough to go live.