The official Arduino site

https://www.arduino.cc/

Arduino.org site open source

Arduino.org

Hardware

Arduino Uno Rev 3 schematic

Arduino reference

Official reference for Programming and Libraries

Programming

Arduino Programming Notebook (40 pages)

Programming

Beginning Arduino Programming - A reference manual for Arduino Programmers (271 pages)

Programming

Arduino Libraries

Where to buy 

Electrokit.com/

Where to buy

Miniinthebox.com/

 Examples and Projects

Arduino Learning (pdf)

 Reference manual

Arduino Reference (pdf)