Here's a nivc book on the histoiry of the harmonica in Australia. It called "A Band in a Waistcoat Pocket." Author is Ray Grieve. It's hard to find, but worth the effort IMO. The ISBN is 0-86819-447-6. I have a couple of other books written in German. I'm sure any of these are "essential" but they do provide enjoyable reading. Good pics too.
