I would be interested to read more scholarly writing on bluegrass, roots and branches. My interest is not from an ethnomusicological or historical point of view, but simply because of a love of the music.

Here's an interesting (not comprehensive, but enjoyable) booklet that attempts to trace some roots and define some phenomena of the music:

The Life, Times & Music Series: Bluegrass by Chris Seymour, 1996, Friedman/Fairfax Publishers.