The Will Pirkle books are excellent, and he did a new edition of the synthesizer one. "Designing Software Synthesizer Plugins in C++: With Audio DSP"

Also, you can learn a ton by looking at various other higher level languages do it: Csound, SuperCollider, PureData are all open source.

Oli Larkin made a brilliant page of resources on github: https://github.com/olilarkin/awesome-musicdsp