I think that’s how it’s working - I’ve got 5 books but I’ve only added a few words from a couple of those. I don’t really know how it would tell that you’d completed a book, unless you never used the ‘ignore’ function 
It would be cool if you could see a list of those books somewhere though.
@Raionus - I find the two check boxes under ‘Find Words’ pretty confusing. It’s not quite clear whether I want them ticked or not (like, if it’s unticked and says “search words”, does that mean it is searching words, or do I need to tick it for it search words?).
It would also be nice if the ‘Filter By’ button stayed blue or something when active, rather than just on hover. Then you’d be able to see visually whether you were still filtering by that book.