[Breakthrough]

I find a lot of ambient music without vocals is good (Aphex Twin, Boards of Canada, Nettless, etc...). Not too repetitive, but the sounds give a nice "mask" to the external world and lets you focus.

I also found an online radio station called "Moving Through Space" which provides some nice background ambiance when reading (esp. science fiction): http://www.live365.com/stations/atombob