Crime Library's article on Sweeney Todd -- Man or Myth? Yes, there was a real Sweeney Todd, he really was a barber, he really was a multiple murderer, and there really was a pie shop.
And, happiness and joy, Peter Haining's 1993 book on the real Todd, supposedly the best one on the subject, is coming back into print, presumably because of the movie. I tried to get a copy of this back when it was first published, but was unsuccessful because it was only in print for about five minutes ... this time it shall be mine! ***happy dancing***