Hello Nico,
If you're talking 'bout "signal," there's a bunch of ways to distort/modify it. I assume you are interested to distort it because you are probably a member of a music band... and you're probably the lead guitarists (just kidding), and want to have sound effects.
In that case, you have to dig some popular diy electronic magazine on how to build sound effect (analog circuit).
BUT... if you want to modify the signal DIGITALLY, then it's a bit complicated. Most probably you end up using DSP (Digital Signal Processor). The basic theory is purely mathematics and I think you have to pass calculus class first to understand discrete time signal processing. But if you want to know a bit about DSP theory, you might find a book written by Proakis & Manolakis (Digital Signal Processing) useful. But it's probably very hard to understand.
If you want to know the effect of real world Resistor, caps, cable etc in high frequency, you might find a book written by Ott (ooh, i forgot his full name) useful (the title contains word like "Noise Reduction...").
But, to be exactly, what kind of distortion you want to perform? If you're looking for signal theory... I mean ONLY signal (no schematics), try to read Proakis & Manolakis book. The book explains SIGNAL in time domain & frequency domain.... All Maths and as you wish... no schematics (yucks!)
Okay, I'll stop. Probably you're having a headache right now of what I'm talking. Btw, for 17 years old kid... able to calculate & build class A amp, you're DAMN GOOD! I don't understand how amplifier works, until I take analogue circuit class
Cheers,