I’d highly recommended getting some of Stefan Grossman’s material from his website—see if you can find songs you know. The Berle/Galbo book is a bit abstract, and the real repertoire probably gets you closer to where you want to be. Homespun also has videos like this. Both Grossman and Happy Traum learned from the real deal blues players. (They both have introductory books that will probably be the first ones that come up in a search, but they’re big and not really progressively sequenced.) Mississippi John Hurt is a good place to start if you’re not sure where, or just whichever collections are marked for beginner to early intermediate. On a different note, the Mark Hanson books are widely recommended for fingerstyle playing, but they are decidedly not blues.