Update: there is a way to edit metadata.
When Apple introduced the iBooks app, and moved ebooks from iTunes, it introduced an annoying constraint. You could no longer edit the metadata of your ebooks. You can’t change the author’s name – which you might want to do to display authors in last name, first name order. You can’t change titles, which you might want to shorten. You can’t change “categories” – or genres – which are often different from what you use to organize your books.
It’s actually ridiculous that Apple doesn’t let you do this. Since your ebooks are more-or-less hidden, you can’t even easily take them out of iBooks and tag them with other apps.
Apple lets you change almost any tags for other media files in iTunes; only books have these limits. There’s no logic to this at all, unless it has to do with the fact that some books can stay in the cloud (see above; two books have cloud icons on their corners), and changed tags could confuse things. Unlike in iTunes, where you can turn off “iTunes in the cloud,” you can’t deactivate this in iBooks.
Those who have a lot of ebooks, and who don’t buy them from Apple, know that they can tag their books in Calibre before adding them to iBooks. But it shouldn’t be this way; tagging is a basic operation that should be available for all media files.