I originally purchased this book on account of its nice chapter on minimalism and its introduction by Philip Glass. Fortunately, I was very surprised to see myself devour the book's chapters on non-minimalist 20th century music as well. The book is well written and well categorized.
The book is essentially split into chapters based on style (in chronological order) starting with the very unknown 19th century American composers. The writing focuses more on the personal lives of the composers and how their pieces were received rather than a theoretical evaluation of style or its evolution. I actually prefer it be written this way since this book was my introduction to American classical music.
The minimalism chapter is good, especially when compared to other multi-stylistic books which only cover minimalism (and usually 20th century music) because they have to.
If I were teaching a class on 20th century music, this would definitely be a required text. Of particular interest is the concluding chapter on the author's views concerning the future of music; he brings up some interesting points worth mulling over.
I didn't care enough about the early American composers too much (Griffes, etc...), so my review doesn't reflect those chapters.
I certainly recommend this book to anyone who is interested in 20th century American acoustic music (electronic music isn't really covered).