How does it compare with Wikisource? It looks like Project Gutenberg is older, but given https://www.gutenberg.org/help/errata.html is looks like the UX (as a contributor) is better on Wikisource.
Projects like Distributed Proofreaders also operate on a top-down basis where there's a project manager for each book. On Wikisource, you can just get stuck in where you like. No one technically "owns" anything.
The Monthly Challenge [1] is quite a fun new innovation where there's a bit more community activity, as a lot of people like to plug away on their own projects, which some people think is a bit boring.
[1]: https://en.wikisource.org/wiki/Wikisource:Community_collabor...