What an excellent project! I would have thought that flattening the books enough to scan them will damage or destroy them. Google developed some software for sc

What an excellent project! I would have thought that flattening the books enough to scan them will damage or destroy them. Google developed some software for scanning curved pages (so as not to damage fragile books) and then flattening the images, but I don't know if that is publicly available. Have you thought about retyping, or having them retyped, as a solution? You could type from photographs of the pages (the page curve wouldn't matter then, so long as the text is legible.) A 150,000 word book might cost you $250 to have typed via, eg Upwork.(Or 50 hours of your time if you can touch type reasonably.) You could QC yourself. You'd then be in a position to typeset and make an e-pub. (Text/LaTeX/HTML -> e-pub is a fairly trivial process; there are open source and free options.) Hope this helps - good luck!