Reddit mentions of Four Musical Minimalists: La Monte Young, Terry Riley, Steve Reich, Philip Glass (Music in the Twentieth Century)

Here are few I've read and would recommend:

• Noise/Music: A History
  
• Ocean of Sound
  
• The Rest Is Noise: Listening to the Twentieth Century
  
• Talking Music: Conversations with John Cage, Philip Glass, Lauria Anderson, and Five Generations of American Experimental Composers
  
• Contemporary Composers on Contemporary Music
  
• Four Musical Minimalists: La Monte Young, Terry Riley, Steve Reich, Philip Glass
  
• Experimental Music: Cage and Beyond
  
• Electronic and Experimental Music: A History of a New Sound

Okay, I know you said music-related, but I think every creative person should go to their local library and check out Steal Like an Artist. I felt like I knew every principle listed, but to see it spelled out and repeated was a great exercise in evaluating how I approach art.

Also, not sure if you're into the theory of minimalism, but this book explores how four of the pioneers of 20th century musical minimalism developed a distinct style.

http://www.amazon.com/Four-Musical-Minimalists-Twentieth-Century/dp/0521015014

Good stuff. Potter provides many wonderful analyses of La Monte Young, Terry Riley, Steve Reich, and Philip Glass.

Also, Free Composition by Heinrich Schenker (text+examples. So, two different books). Not really pertinent to your post but, unless you already have it, IMHO it's a must-have for a music theorist/analyst. Especially because so many people bash Schenker without knowing what he wrote. :)