AVR is the brain that runs Arduino, but you don't need the whole Arduino board to do fun projects. Experimenting with AVR Microcontrollers, from Practical AVR Microcontrollers, shows you how to create a spiffy set of projects that you can build to learn more about electronics, about AVR, and just to generate new ideas for your own projects.
Alan Trevennor will show you how to create a secret panel project, a gadget to drive your pets crazy, a hallway lighting system, and even a small home automation network.