I would recomend the video The Search For Robert Johnson made by (as mentioned above) John Hammond featuring Robert's friends Dave "Honeyboy" Edwards and Johny Shines. A lot of the stories are told first hand by people who knew Robert.
Alan Lomax's book The Land Where the Blues Began is one of, if not, the best books on the history of the blues ever writen. It contains an interview with Roberts mother (he was dead by this point) and he talks about Robert with Son House.
I would recomend these two sources if your interesteda about delta blues history and Robert Johnson.