It depends on how technical you want to get, but some good pedal-specific guides are:
LPB-1: Really basic, but good overview of all components commonly encountered in pedal circuits, then a specific look at the dead simple (1 transistor) EHX LPB-1 linear power booster http://www.beavisaudio.com/techpages/Sc ... ToReality/
Big Muff Pi: A nice diagram giving a simple explanation of what each part does and how altering each section will affect the sound - http://www.kitrae.net/music/Images_Secr ... 0GUIDE.jpg
RG Keen's geofx "technology of..." articles assume a bit more technical knowledge, but it's a simple matter to read wikipedia explanations of common building blocks RG references if you're not sure.
Tubescreamer:http://www.geofex.com/article_folders/t ... sxtech.htm
Fuzzface:http://www.geofex.com/article_folders/f ... fffram.htm
For a general overview on filters, there is a lot of useful stuff all in one place at:http://sound.westhost.com/articles/acti ... ers.htm#s1
For lots of general pedal-related "how does it work?" information, the beavis audio, geofx and amz websites are chock full, and just require a bit of searching.