Paul Oliver's "The Story of the Blues" is probably the most complete overall history, but Jeff Todd Titan's "Early Downhome Blues" is very useful for musicians because he does a musical analysis(though not of the guitar parts) and includes transriptions of about 50 songs--

Lomax was far from the first to record blues, (it had been commercially recorded for a long time before his field recorder had been invented) though his book,"The Land Where the Blues Began" is the best book on the blues ever--Unfortunately, he is not very charitable in his opionion of any blues recordings other than his own--those of us who spent large segments of an otherwise mispent youth playing Blind Lemon Jefferson, Charlie Patton, and the rest respectfully disagree with him--

The truth is though, when all this is said and done, there is no really clear answer to your first question--

The answer to your second question is more or less a debate--for my part, I would say that when you listened to the recordings of Lonnie Johnson and Blind Lemon Jefferson from the Twenties you hear the basis of everything that you hear people doing today, and more besides--

There have been many great players since, but the techique and the ideas that people work with are not changed much--