Well, some of you know my favorites but to Cliff I would reccomend Fred Mcdowell's "First Recordings" that was released last year. Also new last year was a release of Muddy Waters first recordings "The Complete Plantation Recordings."
Both of these were recorded in the field by Alan Lomax, Fred was recorded in 1959, Muddy was recorded in 1940.
Cliff, check out CDNOW.com for other Delta and accustic blues.
BTW, I recently bought a video from CDNOW called "The Search For Robert Johnson" hosted by John Hammond. It's a great documentary with great music.
Good luck.
Gene E