A lot of what constitutes a "good" dulcimer book depends on what you want to sound like when you're through, and whether you want to use the instrument as an accompaniment for singing (and if so, what kind of songs you want to accompany.
Jean Ritchie plays in the very simple, traditional style -- melody played on one string (or a doubled string), and the other two strings as drones, often using a "noter" to play the melody. It's a wonderful sound for the old Appalachian play-parties and ballads. Her book, which is called "The Dulcimer Book," is a splendid, easy-to-understand introduction to this style of playing.
Lorraine Lee Hammond has taken mountain dulcimer playing in a very different -- but no less lovely -- direction. She can make a mountain dulcimer sound like an orchestra, using all the available strings for melody and harmony, and doing some pretty complex finger-picking. If it were me, I would want to start simpler. But that's a matter of personal choice. I agree that in general, the Homespun tapes are very well done and easy to follow (although I haven't seen Lorraine's. Lorraine also has a book, called "The Magic Dulcimer," which is still available and which is very clearly written. Lorraine's dulcimer renditions of traditional Celtic and mountain tunes are breathtaking, and this book helps show you how to do that.