http://dulcimersessions.com is a free “webzine” courtesy of Mel Bay Publications which ran from 2003 to 2012 and edited by Lois Hornbostel. About every other month it featured articles focusing on mountain and hammered dulcimers by well-known experts on the instruments, with musical arrangements to play. There are 51 issues with many musical styles and lessons included. You can view all the back issues at the link above.
Mountain Dulcimer Resource: DulcimerSessions.com