Ther is a book titled Catalogue of Pre-Revival Appalachian Dulcimers that might be of help regarding questions of the evolution of the dulcimer. My understanding is that the instrument has always had some fret structure. Ralph Lee Smith has written an excellent book on the sweet little instrument. I forget the name. I have it at home and I'll check it and post the title. At the risk of sounding like a broken record, again check university LIBRARY web sites as their holdings might be more extensive than a public library. Some universities that come to mind are Berea College, West Virginia University, University of Kentucky, Ohio University, Ohio State University etc., you get the picture. Go to their main web sites, choose their library catalogues and do a ranked key word search and have at it. You can find titles at least and then can request books through your public library Inter Library Loan service, or you can check for any interesting books you may want to buy through Amazon or ABBooks online.