The Number Devil
The Number Devil: A Mathematical Adventure is a fictional book by Hans Magnus Enzensberger involving mathematics. It was originally written in German, but an English translation exists.
Robert, a boy who isn't very adept at mathematics, has Mr. Bockel, a pretzel-loving teacher, for a math teacher. Mr. Bockel apparently teaches mathematics in a dull manner, sometimes giving them hard problems just for the sake of it (similar to Carl Gauss' teacher, Herr Buttner). Robert also has unpleasant dreams and thus a hard time sleeping at home. One night, during a dream, he meets a self-dubbed "number devil", whom claims to come from a place called Number Heaven (or Number Hell, depending upon the mood of the inhabitants, whom are all famous mathematicians). Over the next eleven dream-nights, the number devil teaches Robert mathematics in an interesting manner (often using strange terminology, such as "prima donnas" for prime numbers and "rutabagas" for square roots). On the twelfth night, they go to Number Heaven/Hell and meet a number of famous mathematicians. When Robert goes to class next day, he uses the tricks that the number devil has taught him and solves the challenging problem that Mr. Bockel gives them in a matter of moments, reminiscent of Carl Gauss' performance with his teacher.
The book is written at a pre-introductory level, and explains many basic middle school and elementary school concepts. Some more advanced subjects are touched upon, but not explored.
The book is heavy on number theory.