There are currently two open-source print documentation projects in production that cover various aspects of the Arduino microcontroller.

Arduino Booklet written by Massimo Banzi in an early release can be found here or here (pdf).

Arduino Programming Notebook written by Brian Evans in its first edition can be

• purchased as a 40 page paperback at publishing cost at LULU, or
• downloaded for free as a pdf over at Freeduino.