An easy solution to this would be to add a simple interface into either The Gutenberg Project's library, or Archive.org's library (or both, even). Both have rather huge collections of public domain epub (and other ebook formats) readily available, legal and free.
Easy enough to submit a PR?
Probably? What language is this thing written in? Lemme go see if it's something I know anything about. I'm normally a Python guy (and sometimes a little bit of Javascript here and there).
Okay, question; Is there a link anywhere to a repository or is this thing not open source?