This is, hands-down, the most interesting math book I have ever read:

Who Is Fourier?

Over the course of 13 chapters, the book manages to progress from “1 + 1 = 2” (in fact, you can read that particular page through Amazon’s “Look Inside” facility) through fundamentals of calculus differentiation and integration (without ever using the word ‘calculus’, at least, not that I noticed), and eventually teaches the reader how to perform a fast Fourier transform (FFT) by hand on paper. All of this is done using fairly small words, humorous illustrations, and curious storytelling. Since the FFT is so widely used in audio and video compression technology, consider this book if you don’t quite understand how the transform operates. And refer to it again everytime your knowledge lapses and you forget how the transform works.

The book is published by an entity known as the Transnational College of Lex. I realized from their Amazon listing that they also have books teaching about DNA and quantum mechanics. If I had interest in either of those areas, I imagine these would be fantastic books for getting started on the subjects.