My inspirations for Althea's Math Mysteries include a number of delightful mathy fiction books, including these titles mentioned in the preface to Althea and the Mystery of the Imaginary Numbers:
The Cat in Numberland, by Ivar Ekeland. The cat lives in a hotel, along with all the numbers. There are an infinite number of rooms, one for each number… (For ages 3 to 83.)
The Number Devil, by Hans Enzensberger. Robert dreams about the number devil, who helps him think about math. (For ages 8 to adult.)
Math Girls, by Hiroshi Yuki. The narrator is friends with two girls, one gets him thinking about harder math questions, and he helps the other get more comfortable with math. This is a series of books. (For those who like challenging math, and are comfortable with algebra.)
Surreal Numbers, by Donald Knuth. Alice and Bill are on a deserted beach. They discover a huge stone with writing. It turns out to be math. (For those who like the challenge of complex logic and some very theoretical foundations of a number system.)
I've described quite a few more delightful mathy books at my blog.