I've been reading this wonderful book for the last 2 months or so, and finally finished it on a plane to Florida. The liner notes describe it well: it is a history of the 20th century as seen through the lens of "classical" music. Alex Ross clearly has a deep knowledge of music (not just classical): I learned a lot about the interplay between jazz, classical music, and pop/rock. It was interesting to learn that the Who's song "Baba O'Riley" was titled after a minimalist composer, for example.
This was a beautiful, illuminating read; Ross deserved the MacArthur award that he received for writing it. If you like music, you should love this book.