I am in the process of buying my first mountain dulcimer.  If Bing’s Beginning Mountain Dulcimer book were the only beginner book to use, I would be cancelling my order for a dulcimer.  I would have no idea how to proceed.  In this book, a beginner is asked to go back and forth among many tunings without  a clear explanation as to why.  Most songs in the book are in DAA, some in DAD, some in other tunings. The author cautions that changing tunings often causes a slackening of the melody string.  
Most of the book covers Modes; I’ll admit that this is over my head, but hey, I’m a beginner.  Perhaps I’ll come back to this book once I learn how to hold, strum and pick the instrument (where is one supposed to pick .. in the hollow, half way up, anywhere?).  If, like me, you’re looking for a book on basic techniques and some nicely arranged beginner arrangements in DAD, this might not be a good choice.  (But I still think you’re great, Bing!)
Note:   In DAD, the book represents an open melody string as D5 (Loch Lomand).  Other books say that same string is D4.