Murderous Maths. Also the Number Devil and Penderwick the Mathematical Cat.
I bought our first Murdrous Maths book on Amazon for about $5 ($1 plus $4 shipping). They aren't published in this country, so you have to buy them used from a place like Amazon or you can get them new from horriblebooks.com.
My mathy ds also liked Horrible Histories and Horrible Science, some graphic novels, the Lemony Snicket books, Garfield comics, Calvin and Hobbs comics, parts of Reader's Digest, and Science Illustrated magazine.
