Best instruction material?
Dulcimer Resources:TABS/Books/websites/DVDs
Steven, there are several different ways you might approach this.
You seem to be looking for a single book. Yes, there are several good beginner instructional books out there, but they tend to teach a particular style of dulcimer, and I would think a true beginner should be exposed to different styles so you can better choose which one is right for you. Wally has already mentioned Jean Ritchie's The Dulcimer Book. For an introduction to a different style of dulcimer, I would recommend Joyce Ochs's First Lessons Dulcimer (Mel Bay, 2002). Even brand new it only costs $10. I have a copy I would send to you for the costs of postage, but you can probably get it faster and almost as inexpensively through Amazon.
Ken has suggested you seek out personal instruction, either through a dulcimer group or online lessons. If you are a true beginner and need help just tuning your instrument, you probably want someone in person. But if you can tune your instrument and know basically to keep the string side up, then you might look into Zoom lessons. A lot of instructors teach online very successfully. Just ask someone whose style of play inspires you.
Another possibility is through an online subscription service. There are two options I know of: Stephen Seifert's Mountain Dulcimer A to Z and Steve Eulberg's Dulcimer Crossing. In both cases you pay a monthly fee and have access to a wide array of instructional materials. It will take some effort on your part to poke around the sites to find the right material for your interests and your playing ability, but both of those online resources will have a lot for you both now and as you progress in your playing.
And finally: dulcimer festivals! Some are in person and some are virtual through Zoom. All of them have beginner classes, and usually those beginner classes are smaller than the intermediate classes so that you get the personalized attention you need. In the tradition of shameless promotion, let me mention the Redwood Dulcimer Day which is online in just a few weeks and has a beginner string that includes lessons by Don Pedi and Aubrey Atwater. But there are other festivals out there where you can get the instruction you need and also meet other dulcimer players.

says your nepenthes gaya.
, but this appears to have caused enormous confusion
. Just call me an agitator
(or maybe a gremlin).