the number devil, by Hans Magnus Enzensberger.

I bought it as it was recommended by a book review for educational books for children. I felt virtuous for at least a week Well I thought it would stay and gather dust on the bookshelves packed with my childhood books Enid B and loads of pony books. Also it said for older children so forgot about it.
Amazingly (something to do with the yellow cover and a red devil on it) my daughter (8) snuck it of to bed. I haven't read it but apparently so I have been informed with alarming frequency this week though mouth fulls of coco pops "it is all about this boy and the devil and maths!"I had a peek and put it back exactly were I found it in case I accused of snooping.
It is pretty good. I couldn't really get too enthusiastic as square roots etc were never really my thing, but my daughter is hooked and has nearly finished it.