Standard Ebooks (standardebooks.org) has beautifully formatted public domain EPUBs. There are also 70K+ free books via Project Gutenberg and similar sources.

I built an app that bundles access to 1.5M+ free public domain books from multiple sources, with OPDS support for Calibre/Kavita if you self-host. Free on the Mac/iOS App Store: getbookshelves.app

Cool project. The reading position sync problem is real.

I’ve been building an EPUB reader for Mac and iOS that syncs via iCloud. Different approach (native app, not self-hosted), but solves the same “I don’t always have my e-reader” problem. Also has OPDS support for connecting to Calibre/Kavita servers.

Not open source but free to try: getbookshelves.app