I've wondered about this myself and have found that the most common responses usually reference, "The Art of Electonics". Great book, sure, and very heavy on th

I've wondered about this myself and have found that the most common responses usually reference, "The Art of Electonics". Great book, sure, and very heavy on theory. There are a few books out now that touch on the Arduino platform - the Make/O'Reilly site has a few. Additionally, there are a couple of folks using a Sanguino/Arduino for building a rep rap, so it might be a good start. I've found "Electronics - Self Teaching Guide" by Harry Kybett to be a decent introduction to electronics in general. I also picked up "Embedded C Programming and the Atmel AVR" which is regularly recommended for the AVR platform.