I’ve heard about Arduino for years now, but never invested any time into using it until recently.  It’s pretty addicting and it isn’t all that difficult to get started with.  There are a ton of examples on the Arduino website (basic circuits and code) and the programming language is C.

If you’re looking for a good book on the subject, I would recommend the Arduino Cookbook published by O’Reilly.  I bought a couple books, but this is the one you really need.