This book by Paul Scherz goes beyond the realm of pedals and deep into electronics.
Related posts:
How to Modify Guitar Pedals, by Brian Wampler
Make – Analog Synthesizers
Vacuum Tube Amplifier Basics
Encyclopedia of Electronic Components Volume 1
How to Diagnose and Fix Everything Electronic
Electronic Projects for Musicians
Handmade Electronic Music – The Art of Hardware Hacking
The Basic Soldering Guide Handbook