My son also enjoyed Number Devil. I would HIGHLY recommend the Murderous Math Series for the kid who is past all of the arithmetic, but not quite ready for Algebra. They are British and the British sense of humor is reflected in the books which my son loved! I tried for months and could not find them used so I broke down and bought the set. I have to say though that my DS8 read the set in one week and continually goes back to them just for fun. I would also recommend the book, Phantom Tollboth.
The game, Equate is also good.
