Can't You Hear Me Calling: The Life of Bill Monroe by Richard D. Smith---a straightforward, comprehensive bio by a man who knows the field.
The Monroe Reader, edited by Tom Ewing--an annotated collection of journalism about Monroe from the late 30s until the end. Read as a whole, it gives a great picture of the changing role of grass---from plain old country music, to revived "folk" form, to hippie fascination, to middle class musical appetizer.
Bluegrass Breakdown: The Making of the Old Southern Sound by Robert Cantwell---a book length essay on "what it all means," written in Cantwell's sometimes beautiful, sometimes maddeningly impenetrable (and wordy) style.
I haven't read it, but Tom Piazza's book on Jimmy Martin--True Adventures with the King of Bluegrass--has been praised highly. I've heard it described as a wildly funny, true to life version of the touring circuit, shown through the life of a real wild character.