The Homespun dulcimer tapes mentioned above are REALLY worth the money... I have Lorraine Lee Hammond's teaching tape set that comes with a book with tablature.
In a way,they are like having many, many, private lessons, from a dulcimer master, for much less money than actual lessons would be. You can go at your own speed, play things back over and over again, hear what you're aiming to sound like....and the Hammond tapes take you from beginner lessons, to changing modes, to harder things like chording and sliding (with a slide).
So, that's my recommendation. I think books alone can be quite helpful, (I love Lorraine's 'The Magic Dulcimer' as a beginner/intermediate book in addition to the others already mentioned), but to hear the sound of the songs you're working on, presented in a clear organized fashion,is going to take you way farther, imho....