While Manybooks have a greater selection, I completely agree with HarryT in that Feedbooks have superior formatting. I started checking it out on saturday when I got an iLiad, and have already read two books from there. A brilliant site!
With Manybooks I'll have to do quite a bit of formatting to get the book readable at all.
I don't know if Hadrien or anyone else from feedbooks will read this, but I have a few suggestions (nitpicking really). They seem quite responsive to suggestions from their users from what I'm reading here
I searched the forums, but if there already is a thread about this it may be in French, or not very active.
Search: It's a little confusing. I searched for Clarke (Arthur C.), he has written a few stories that are online legally. I got hits for Peter Watts among many others, probably because of his novels featuring Lenie Clarke (which are good, by the way). Maybe an advanced search function? Or, at least, when I select "Author" the search will be for the name of an author, not just return author nodes (It seems that you're using Drupal).
Word count: When I want to check out an author I've been recommended I frequently want to check out his/her short stories, if any, to see if I like the style. A categorization might also work, but a word count is easy to develop, cheap on the db (count and store only when updating the book) and would be helpful.
Custom PDF bug: There seems to be a bug; when I change preferences I will frequently get the previous, cached version. Other times I will get a freshly generated one with the new settings. I've experienced this when changing font sizes and fonts. I see the value in caching, but people will rarely be requesting the exact same custom PDF twice.
Clickable PDF TOC: This might be more complex, since you'd need to generate the PDF first, then TOC, and merge them. It would nevertheless be a valuable addition, and one I'm sure that you have already thought of.
Floating point values: Again for custom PDF. There is a difference between font sizes 8 and 7.5
This might be an issue with your generator, but FP values for font size is supported by most software.
Names: What's up with the surname-only policy? A very small nitpick, but my monitor has room for given names as well
It looks very strange, in addition given names can be an advantage when scanning long list for a specific person. Or maybe it's just me.
Again: this is mostly nitpicking, I give these tips because I really like the site. I understand it is under constant development as well, maybe my requests are already in the pipeline.
Cheers, and keep up the good work!