Boulanger and Lazzarini "The Audio Programming Book" - great if you want to start working straight away in C

Boulanger - "The Csound Book" (another classic language I still use today)

Greenbaum and Barzel - "Audio Anecdotes". A fascinating series of 3 volumes with all sorts of wisdom on FX design, studio systems and composition

Wilson, Cottle and Collins - "The Supercollider Book". In the style of The Csound Book, but with SC.

Loy - "Musicmathics". A rare and much under-rated two volume set on the equations behind audio DSP

Bilbao - "Numerical Sound Synthesis". A hard but rewarding journey to understanding audio physics as linear diff systems and implementing them efficiently in C, Goes well with Perry Cook's stuff.

Benson - "Music a mathematical offering" Very unusual book that analyses many subjects in music physics. Equations but no code.

Miranda "Computer Sound Design". More about music synthesis than "sound design" imho, but has some interesting fringe methods like cellular autonoma and genetic algorithms.