Wald is far, far more knowledgeable about 1930s blues than pre-1915 blues. I first talked with him about the distortions regarding pre-1915 blues in this book in 2004, and I had another detailed discussion with him recently, in which imo he had nothing to defend some of what he's claimed (about pre-1915) than repeated handwaving.

I'd definitely _not_ recommend Wald on pre-1915 blues (but would highly recommend him on Robert Johnson in particular).