Musimathics is a guided tour of the mathematics of music, fully up to date, covering both the classical subjects and cutting-edge developments. In two volumes, published by the MIT Press in 2006 and 2007.
Volume I covers musical scales, instruments, acoustics, psychoacoustics, composition and music programming.
Volume II covers digital audio, filtering, acoustical systems, sound synthesis, the Fourier transform and its applications such as MPEG audio and MP3.
Musimathics is available from your local bookstore and from the MIT Press, Amazon, Barnes & Noble, Best Buys and many other locations.
An excellent review by Mark Sealey appeared recently at The Classical Net.