Official site and documentation
Arduino reference
Tutorial/Examples
Oficial Tutorials
Arduino blog Blog with new projects based in Arduino platform
Arduino Playground A lot of community examples and tutorials
LadyAda Tutorials A lot of lessons by LadyAda
Make Blog Crazy projects and ideas
Books
- Getting Started with Arduino, by Massimo Banzi
- The Art of Interactive Design, by Chris Crawford
- Physical Computing: Sensing and Controlling the Physical World with Computers, by Tom Igoe and Dan O’Sullivan
- Making Things Talk: Using Sensors, Networks, and Arduino to see, hear, and feel your world by Tom Igoe
- Programming Interactivity by Joshua Noble
- Making Things See by Greg Borestein