I highly recommend anything by Stephen Seifert. He has several book/DVD sets that are well-developed and clear to understand. To begin with I'd recommend his Foundational Techniques book/DVD or his series of 13 lessons, each containing a book and DVD. The lesson series takes the learner from a basic arrangement to increasingly more advanced arrangements introducing new techniques that can be incorporated into any song. Stephen also has Jam Books available for the two most common tunings: D-A-A and D-A-D. They are excellent for expanding your repertoire once you become familiar with dulcimer tablature.
Another excellent choice for beginners would be Maureen Seller's Teaching Books. She has produced two volumes in D-A-D tuning and has adapted her teaching books to produce the First Instruction Book for McSpadden Mountain Dulcimer
. The McSpadden book can be used with either D-A-A tuning or D-A-D tuning and includes a lot of material that is overlooked or skimmed over in many of the other instruction books. It is furnished free with the purchase of a McSpadden dulcimer, but I suspect it can be purchased separately, if desired. In my opinion, it is one of the better beginning instruction books on the market.
Here's the link for Maureen's McSpadden edition.http://www.mcspaddendulcimers.com/searc ... =10&page=7