Archie's Barbershop in Washington DC:
http://acousticblues.com/

If you can make it some Saturday afternoon there, you will learn more than you can imagine (and they are very welcoming to both beginners and outsiders). A trip to DC on some weekend would be well worth your while, if you live in the Eastern US.
If you can't make it, follow the links from the site.

--Charlie Baum