There's something to be said for groups like this one and Standard eBooks (mentioned elsewhere in this thread) doing some work to clean up and beautify public domain books like the ones in Project Gutenberg. Even Standard eBooks mentions that their books are all sourced from Gutenberg.

This group seems much less complete than SE, and in my opinion, they're not nearly as nice looking. I think that's more a matter of personal taste though.

The difference is UX and aesthetics. Planet eBook is a more pleasant experience if you're a casual consumer browsing around for a well-known book to read, while Gutenberg's site suits someone with more specific needs wanting to explore the long tail of public domain content.

To borrow a DevOps analogy, Gutenberg's website handles books as cattle, Planet eBook treats books a bit more like individual kittens insofar as they're hand-picked with (presumably) humans selecting the extracts and covers. The Gutenberg site has an early 2000s programmatically generated UI aesthetic that is similar to Internet Archive and the Wikipedia homepage. (At least in the case of Wikipedia, I've heard there's considerable community resistance to any significant UI changes - it's not just a budget constraint.)

A UX example. Search for "1984" on Gutenberg doesn't yield the book of that name. https://www.gutenberg.org/ebooks/search/?query=1984&submit_s.... Planet eBook site does: https://www.planetebook.com/?s=1984

To be clear, Gutenberg has been an incredible resource and gift to society. It's just that there's a need for custom front-ends to match different needs.