The Number Devil: A Mathematical Adventure is an illustrated story about how weird numbers can be. A boy is tormented by a teacher who gives boring math problems in school and then is visited in his dreams by a number devil who introduces him to prime numbers, factorials, number pyramids and a host of other math concepts (but with different names, which would probably get confusing if you were trying to use this as a textbook).
The introductions are not at all in-depth; they mostly focus on how weird it is that numbers can do these things. So it’s probably a lousy textbook. And it’s not really much of a story either. What it is is a kind of children’s Socratic dialogue about numbers. No real explanations are given but intriguing hints about the depths to which you can explore Fibonacci sequences and the like litter the pages. It’s like a field of rabbit holes for someone to get intrigued by and then fall into by getting answers (such as they are) in other books.