I'm currently studying sound design, however over the past couple of years i started creating audio effects/instruments. I have also been programming in C++ for quite some years. I noticed my shortcomings though; i can design compressors, envelopes and simple stuff like additive synthesis quite well along with basic signal knowledge (like samplerates, nyquist theorem), however i don't understand the theory behind some of the more dense mathematical subjects like filter designs, transfer functions, fourier transforms, complex numbers, time/frequency domain etc., or even the mathematical notation of such.
Without sounding too oblivious, what are the relevant fields for educating one in such topics? Also, if anyone has some book recommendations for easy introduction to required math/basic signal theory i would be most grateful :)