Free Arduino reference guide
Erik Verberne teaches Information and Communication Technology (ICT) in Netherlands and looked into the Arduino platform in an attempt to enhance his department’s curriculum. He diligently documented his research and is now sharing it with the community. The result is an e-book of over 200 pages full of examples and tutorials, from Arduino and ATTiny programming tips, to working with LCD and LED displays, playing sounds, detecting obstacles, wireless communication via IR and Bluetooth and much more. See the official thread in the Arduino forum for the latest update and a copy of the Arduino guide and make sure you post a comment there if you find it useful!