It's rare that I recommend music, but I've recently decided that Brian Eno's Music For Airports is some of the best background music for writing.
I've owned this particular CD for at least 4 years but have only recently begun to listen to it with any regularity. A couple weeks ago, I popped it into the CD player while hacking on a book chapter and left it on repeat. Since then I haven't even thought of taking it out of the rotation. Like much of his other ambient work, its very subtle and just fades into the background as your concentration and attention focus back on the task at hand.
If you're a fan of Brian Eno or Ambient Music in general, I highly recommend it.