Have you tried the "Murderous Maths" family of books? They are non-fiction and filled with humor. Our kids (twins, now 13yo) read and reread them many, many times over the years. In addition to math, they have series on science, history, geography, etc. They are published (by Scholastic, I think) in Britain and not widely available in the U.S., but easily obtained by mail order on the web.