Reddit mentions of How to Modify Guitar Pedals: A complete how-to package for the electronics newbie on how to modify guitar and bass effects pedals

There's the Brian Wampler book that people always talk about in this sub, I don't know if it's exactly what you're looking for, but you should check it out anyway, it's a great source.

I think it is this book Splunlen is refering to: https://www.amazon.com/How-Modify-Guitar-Pedals-how/dp/1434801063

Please note that there seems to be a new version out soon.

If you put in the time and effort you can pick it up fairly well. If you really want to get into design and modding and stuff I'd say find a few fairly simple circuits and socket a bunch of the components so you can see how changing various parts impacts the sound. This is supposed to be an interesting book from Brian Wampler. Most of the negative reviews basically bash it for basically taking informations and schematics you can find online and putting them into a book, but the analysis and organization helps you work through some of this stuff. This is another good one too from Craig Anderton. The Anderton tube sound fuzz is the basis for one of my got to distortions.