I've read the book, and I'd say Stagolee still has great relevance. Certainly the author thinks so!

It's very good on the history, not so great on the interpretation. In other words, he shows a lot about who the original person was on whom the song is based, and makes a very convincing case, but he could have developed the analysis more in terms of what the song means to people. Still, by and large it was a good, groundbreaking book.