I've been working on a little synthesizer as a means of learning rust. https://github.com/jimktrains/synth

Sound on sound has a series on synthesizer concepts. https://www.soundonsound.com/series/synth-secrets

Dsp guide is a great resource. http://dspguide.com/

Music dsp also has some good information and implementations. https://www.musicdsp.org/en/latest/index.html