I'm just finishing this book, and I echo the recommendation. In fact, when I bought the book, I didn't know it was about Zettelkasten. This book won't satisfy those looking for concrete examples, but it will give you a lot of specifics and whys. Obviously, I'm in the early adopter stage at the moment, but what I'm reading fits my problem space (tons of notes, but I can't seem to find what I want, and I re-do research periodically because of it).