Can anyone recommend a good instructional acoustic blues covering the different styles including piedmont and delta?
UM I can recommend good acoustic blues songs.....but truely the best way to learn is your ear...thats how I started.I first tryed "Kind heart'ed woman" by robert johnson,thats a good 1 to start on.
YOu mean one song that phases from Piedmont to Delta? Don't think there is one, but I can give you some good songs to play, because I love the blues!

"Hey Hey" Eric Clapton (unplugged album)

"Never Far Away" Jack White (from Cold Mountain soundtrack)

"One Bourbon, One Scotch, One Beer" George Thourogood (originally an electric, but it sounds cool on an acoustic too!)

"Layla (acoustic)" Eric Clapton (From unplugged album)

Hell, Clapton's whole unplugged album is good.
Anyway, those are only ones in standard tuning, if you don't mind using alternate tunings (Open G ftw!) then I can give you some of those too.

If you are just beginning fingerpicking and stuff, then the Jack White song listed above would be a good place to start. I even have a good tab for it if you can't find a satisfactory one.

Hope I helped,

EDIT: Oh, as previously stated in the thread, learning by ear is the best way to learn, only use tabs if you can't get it at all. But then, I have been guilty of using them from time to time