Check out the 3rd Edition.
Written by Massimo Banzi (the co-founder of the Arduino Project), this nice little handbook walks you through the open-source hardware/software project that is Arduino!
Hear of the Arduino / Freeduino yet? It's an open-source hardware platform for the Atmel ATMega168 microcontroller, plus the programming system that runs under Java for Windows/Linux/Mac. It's gathering a huge following, being open-source and hacker-friendly.
If you're new to the scene, you may want to check out this cool little handbook by Massimo Banzi (the co-founder of the Arduino Project). Getting Started with Arduino gives you a solid introduction to this platform, so you'll soon be up and running with your own projects!