Would be nice to help out creators of ebooks and check for broken links. That seems to be a real problem.
There is a tool called epub-linkchecker, but development seems to have stalled. I don't know much about epubs, but it looks like it's some XML-like format with normal <a> tags, so it might not be too hard to parse (either with our html link extractor or the plaintext extractor).

If there are any people interested in trying to add support, add a comment here. We can provide some guidance if needed.