The editions of this book seem to come out about every 10 years. 1st was in 1975, 2nd 1986, 3rd 1996. We could have to wait a couple of years. I'm going to borrow it on interlibrary loan.

The publisher of the 2nd & 3rd ed. was the Country Music Foundation. They don't seem to have a web presence, but here's what I found in a white pages search:
Country Music Foundation
222 5th Ave S
Nashville, TN 37203-4206
Phone: (615)416-2001