Robert Johnson's Official Site
This site is maintained by the estate belonging to Johnson's son Claud Johnson, who finally got possession of Johnson's estate nearly 60 years after his father's death.
The Legend of Faust
Here's an interesting and authoritative website entirely devoted to the legend of Faust and its offshoots.
Frank DiGiacomo, "Searching for Robert Johnson," Vanity Fair (2008)
Here's a great story on Robert Johnson's mysterious life and death, complete with interviews.
Jesse Gress, "10 Things You Gotta Do to Play Like Robert Johnson," Guitar Player (2010)
Here's some advice from Guitar Player on how to imitate Johnson's deceptively complex guitar playing.