Hi, this topic is from a while ago, but I'm new here so thought I would chime in - I have a copy here of "The Dulcimer Book" by Jean Ritchie, Oak Publications, 1974. Actually it is about to be overdue to the library - I rather not return it! Before reading this, my knowledge of the dulcimer was pretty much that I own one and mess around with. This book gave me a valuable background on the instrument and the musical tradition it comes from. Plus she describes tunings for different modes, and it is a song book.
Two things I thought were interesting to mention about it are;
The full title is actually "The Dulcimer Book; being a book about the three-stringed Appalachian dulcimer, including some ways of tuning and playing; some recollections in its local history in Perry and Knott Counties, Kentucky; some observations on the probable origins of the instrument in the old countries of Europe; with plentiful photographic illustrations and drawings; and with words and music for some sixteen songs from the Ritchie Family of Kentucky" (!)
At the same time I got this from the library, I check out another book called "The Appalachian Dulcimer Book" by Michael Murphy, Folksay Press 1976. I note the dates because this later book contains the most blatant plagiarism I've ever seen, with entire sections of Jean Ritchie's book repeated without changes, and in some other cases slightly paraphrased. So I returned it and stuck with the original!