Along with the 6 demons who's books were burned by Kiyo/Gash (the ice kid, the plant kid, Koruru, the clown, that big guy's evil partner, laser guy) Sherry and Brago have burned 2 books that we've seen (the dog and that beastly yeti thing). With 70 out of 100 left, that means a whole 22 demons were defeated offscreen. I bet at least a couple more of them were beaten by Sherry/Brago. Regardless, this also means that most demons haven't burned any books.