Tony Russell is one of the most knowlegeable people around in respect of the blues and early country music. I commend to you a reading of his 'The Blues from Robert Johnson to Robert Cray' Harper/Collins. His recommendations of 'Milestone recordings' should set out a pretty good guide for you to be getting on with. Lawrence Cohen's (Ed) 'Nothing But the Blues' Abbeville Press also has a wealth of information.

--Stewie.