I picked the Jack Orman ebook Guitar Effects Explained from Amazon today. So far I am finding it very readable. It avoids too much math, and reads like a plain English description of how effects work at the circuit level.
I highly recommend this to anyone interested in building pedals, and also to anyone who just wants to know more about the magic.
There is some assumed knowledge required to understand parts of it, but nothing so far that a quick Google search can't fix. For example, the early section on boosters says "our signal is AC coupled into the booster so the clipping takes place symmetrically". Jack doesn't explain what coupling is though, or how it leads to symmetric clipping in the gain stage. Hopefully he explains the related role of the coupling capacitor after the gain stage.
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