You also need a system to manage books coming in, separating spam/viruses from real content. You either need a centralized source verifying books (zlib and usenet approach) or some kind of voting system like thepiratebay. If it's a centralized source, that'll be the weak point to take down, and voting systems won't work well for unpopular books.

Last I checked there were some pretty good usenet servers flying under the radar with even more books than z-lib, manually curated. So there might not be enough demand for your idea at the moment.