Since I have fought my way in the course of time through various publications, I do not want to keep the experiences to myself. The books listed are selected from my collection, from any area I have mentioned some, which appear to be particularly useful and motivating in my mind. Remember that too much material often is no benefit, because you just jump around.
I want to start with the "Harpbuch" by Mario. It is no harp school in the sense of "music book with rising claim" but a fundamental work for all harp players. Structure, function and maintenance of the harp, the necessary music theory and finally playing techniques with song examples, all this is clearly presented and illustrated generously.
At www.harpbuch.de you can make yourself a more accurate impression of the content and listen to the sound samples.
A good starting combination with this work would be the publications of Perry Letsch.
A well-built school, especially if one has suffered from traditional german music lessons. Starting from most known songs, you will slowly but surely be guided towards folk and blues.
"Do not judge a book by its cover" said Willie Dixon. The website of Perry Letsch: www.perryletsch.de
For absolute beginners "Mundharmonika spielen" is the right start. If you already have a little musical experience, you can also cope with "Bluesharp" as a beginner booklet. The Songbook is always a recommendation.
The second major division for me is David Barrett. There is a bewildering number of publications at MEL BAY, some with similar titles. For someone who has already mastered the basics with local songs, I would suggest the level 2. Here are three works that I have found useful.
Do not be alarmed, the booklet "Licks Chart" has no pages, it's just an envelope with CD. It is reasonably priced and is also suited to get familiar with the way of David.
There is also a website: www.bluesharmonica.com
To apply what you just learned, a PlayAlong booklet is recommended. You want to play like your role models, a bit at least, and therefore such resources are essential. Not everyone lives next to a blues club.
Here are three very good copies, two books of Hal Leonard and the CD of Olaf Böhme: http://bluesclass.de/
Although Adam does not offer a schoolbook, but his videos on YouTube and his purchase videos on modernbluesharmonica.com I found very motivating. Therefore, he must be mentioned here necessarily. Do not be chilled by the talking. You can learn a little English. If the vanity of the self-proclaimed Doctor becomes annoying or if he only talks about tech: just do not listen! He really has a lot of useful suggestions and is also entertaining.
Paul Lassey has a channel on youtube with beautiful exercises, for which you may not necessarily know how to speak French. For the french folks among us, there are many information and offers on his site: http://www.apprendrelharmonica.com.
Paul also manufactures precipitated custom harmonicas made out of noble materials.
Challenging, but as far as I can tell very instructive, are the books by Steve Baker. Here especially, the expression will be trained. You can safely buy Volume 1, even if you are not professional.
This booklet was maybe good in former days, because there was nothing else. Meanwhile, it has become obsolete, especially the sound examples. In the forum, I wanted to give it away for postage reimbursement and got criticized for :-)!