Three essentials would be "The Friends of Charley Patton" (Yazoo), and the Muddy Waters "Plantation Recordings" before he headed north.
I hear that there is a new CD version of the Son House backroom session also recorded by Alan Lomax (I have an old one).
I think the Newport Festival "Blues With A Feeling" collection is also special (can't remember the record company, Vanguard, maybe?) yours, Peter T.