Art is always in dialog with the time of its creation, so this book is looking at 20th century music. But in saying that, you have to understand that it is so-called “classical” music that the author is looking at. That may be a bad term to use, since there is a technical meaning of classical that refers to a specific music of a time (Mozart and Haydn are classical, Mahler definitely is not), so some people use the term “serious” music instead, but I happen to think that the music of John Coltrane is every bit as serious as anything by Stravinsky. But if you decide to pick up this book, it does help to know what the subject matter will be. If you are expecting more than a passing glance at 20th century music like rock’n’roll, you will be very disappointed. Nonetheless, all kinds of music are in dialog with their times, so you can draw relationships if you look closely. For example, the book mentions that Stockhausen was an influence on The Beatles, and appears on the cover of Sgt. Pepper’s as a result.
20th Century music is profoundly influenced by 2 World Wars, the atomic bomb, genocide, Fascism, and Communism. One major result is the move away from tonality, which has proceeded to the point that by the 1980s it had become almost impossible to secure any academic position if you were not a 12-tone composer. But this also meant a move away from popular acceptance, since 12-tone compositions are almost by definition impossible to listen to for the average listener. But if you are curious about why composers did what they did, what their aim was, and how they lived in this dialog with their time, this is an excellent book.
BTW, I listened to this book as an audiobook purchased from eMusic.com. They offer audiobooks in straight MP3 format without any DRM, which is why I like them.