What to read after Folk Style Autoharp?

From the introduction: "It has been the author’s attempt to present, in a concise form, an introduction to the country style of playing the Autoharp. The styles presented have been kept as general as possible so the student can apply the techniques to any specific artist he wishes to study. Once the student has mastered the material in this book, he will have no difficulty in studying various traditional artists, or, on the other hand, supplementing his repertoire by his own arrangements and compositions.

The major difficulty in preparing a book such as this is to invent a notation that is easier to read than the autoharp is to play. I have retained the musical notation because it gives the “ shape” of the melody. In addition, music reading skill will be necessary for working from other printed sources. The music is supplemented by simple notation which indicates how the autoharp is to be played. I would like to thank Mr. Mayne Smith for fruitful discussions in regard to the notation used in this book."