As far as resources for musicians hoping to learn something from Robert Johnson, there is a book out by Woody Mann which has music to all of of his tunes (one edition even has guitar tab) and words to most of them (but strangely enough, only one verse of Sweet Home Chicago). It is called THE COMPLETE ROBERT JOHNSON. I can recommend it highly.

One of my favorite RJ songs is Love In Vain. I suspect that he is the originator of that song, for at least three reasons. First, the device of adding a fourth line ("All my love's in vain") to each verse is unusual (I can't recall any other examples off the top of my head). Second, the chord progression is a little non-standard for blues. Third, I've never heard a different contemporary version. I like the song because not only the accompaniment but also the lyrics are so sparse--what is left unsaid is just as important as what is said. I get the idea that the singer has just admired the girl from a distance and she's leaving on the train without ever even knowing that he loved her.