For anybody who is interested in the origins, growth, and influence of California country music on America, I highly recommend "Workin' Man Blues", the definitive book on the topic.
The book talks about the various forms of country music (most people with experience in the music plot out 10 separate genres: honky-tonk, oldtime, bluegrass, gospel, rockabilly, Bakersfield sound, Nashville sound, Cajun, Texas style, and of course my favorite, Western swing). Here's a link to the genre entries:
http://en.wikipedia.org/wiki/Category:Country_music_genres
Here's a link to the book's author:
http://www.geraldhaslam.com/wmblues.htm
RADIO: Radio was a huge influence in the spread of country music in California, especially in the Central Valley (KTRB, et al), and the Bay Area (KEEN, KSAY, KVSM, KGO, etc.) The book notes that the spread of the music came about largely due to the migration of people here from Oklahoma, Texas, and the like, and the unifying factor radio had in bringing these people together after having been uprooted from their ancestral homes.