If you like the genre,the most insightful and intelligent book on progressive rock that I've read is "Rocking The Classics" by Edward Macan. It's not a biography of the bands,or anything like that-it's a somewhat scholarly book on the progressive style and the way it correlates to classical music.
I spent YEARS listening to that stuff. ELP, King Crimson, Weather Report.
Still trying to find Lark's Tongue in Aspic