The Number Devil, by Hans Magnus Enzensberger

Subtitled "A mathematical adventure" -- and what an adventure this book is. Aimed at children, the story (in which twelve-year-old Robert dreams nightly about a "number devil" who shows him the wonders of mathematics), is a wonderful introduction to the magic of mathematics, nicely illustrated by Rotraut Susanne Berner. It reminded me (40 years on) of many of the things that I found enticing about mathematics at about that age. (Or actually a bit younger, since I was of course a prodigy!)