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.