Murderous Math, Cat in Numberland, The Number Devil there are bunches-
http://hoagiesgifted.org/mathematics.htm

@Peter- my son read Percy Jackson, followed by the two Kane Chronicles books. He then jumped to 39 Clues (many also by Rick Riordan). He's now happily into The Alchemyst. It was recommended by the Barnes and Noble guy as 39 Clues meets Harry Potter. So far, he's dead on! There are many of those as well in the series.