chiming in here with a "huzzah!" for the Murderous Maths series. Our DSnow9 just devoured them ... he was 8 at the time. We were incredibly fortunate in that our alternative public school actually had 6 of the books in their library - but we did order several from

www.half.com

search Kjartan Poskitt (author)

and you can usually find them for $3-4 plus shipping - we were able to bundle some of the shipping, too.

This is a fabulous site for lots of other stuff, as well:

http://www.fun-books.com/mathematics.htm

this one is too:

http://www.horriblebooks.com/

hope this helps!