Math and Music theory books

There's Music: a mathematical offering by Dave Benson. It can be downloaded from his website.

There's Philip Ball's the Music Instinct, although this would be more from the science point of view than the mathematical one.

If you like category theory and topos theory you might want to look at Mazzolas, Topos of Music: Geometric Logic of Concepts, Theory, and Performance

I don't know which level you mean, but Mathematics and Music seems nice. There is also Musimathics, which seems more advanced. [Disclaimer: I don't have first-hand experience with either book.]

Besides the ones already mentioned, there is A Geometry of Music: Harmony and Counterpoint in the Extended Common Practice by Dmitri Tymoczko, which takes an orbifold approach.

If you want to understand scales from a mathematical/dsp perspective, and why a certain scale is the most "natural" for the music of a given instrument or culture, you should check out Tuning, Timbre, Spectrum, Scale by Sethares.