@Bitrot Just search for the book you want, and if it isn't on your instance, it will give you the option to import it from other instances, Inventaire or OpenLibrary. Then you'll be able to edit the details. If none of these options are the right ones, you can manually add the book by clicking the last option.
But indeed, it does have a lot of missing books and editions of books. If you do not have the patience to add them, then it is clearly not for you.
Inventaire is also AP enabled, despite being centralized, tho.