The books already cited, The Story of the Dulcimer and Appalachian Dulcimer Traditions by Ralph Lee Smith and A Catalogue of Pre-Revival Appalachian Dulcimers by L. Allen Smith, are the core works for any dulcimer historian's personal library. A fourth book, Wilfried Ulrich's The Story of the Hummel, is essential for understanding the parallel history of the instrument in Europe. Wilfried goes into detail describing the history of the fretted zithers, including instruments that pre-date the Praetorius scheitholt. Wilfried argues that the instrument has many names depending upon the country in which it is found. The book is available in an English language edition for those of us not fluent in German. I obtained my copy directly from Wilfried through his website. If still available, it is well-worth the investment.