The original XP website is still online (http://www.extremeprogramming.org/) and worth a read.

The main book was Extreme Programming Explained: Embrace Change by Kent Beck.

Both are really good resources for understanding agile, where it came from and why things are done. A lot more emphasis on the engineering practices that enable agility in both compared to scrum and other recent agile processes.

never forget:

http://wiki.c2.com/?CthreeProjectTerminated