[bloat]

If you are interested in classical music - here's a couple of good books on the 20th century.

  "The Rest Is Noise" by Alex Ross
  "Modern Times, Modern Places" by Peter Conrad

The first is just music, the second one covers art and culture in general but has a lot about music.

"America's Musical Life: A History" by Richard Crawford is also great, focusing on America obviously and covers more than just the 20th century.

[autarch]

I second The Rest Is Noise recommendation. I really enjoyed it. Make sure to check out the companion website, which has tons of samples of the music he discusses.