Modern Sheet Music/Tab?
Dulcimer Resources:TABS/Books/websites/DVDs
@geezer , I agree with what Brian G advised. With your experience, there really is little need for you to be looking for tab books. Though dulcimers are not tuned to the same intervals as guitars, thus you'd have to learn different fingerings and chords than you use on your guitar. But you should be able to figure out lots of contemporary tunes you already are familiar with, on your own.
If you read sheet music, you can skip looking for dulcimer tabs books and instead do google image searches for tunes you want to play. Keep in mind that if a tune is contemporary, it will more likely be under copyright and this limits your ability to get copies of it for free and/or online... whether it's dulcimer tab, standard sheet music, guitar tab, etc.
@Gennaro - there are skilled dulcimer players around who play complex modern music, blues, baroque music, etc on their dulcimers, and they do it beautifully. But do keep in mind that mountain dulcimers were created in the 1800s in imitation of yet older European folk instruments that were used by 'common folk' to play relatively simple folk tunes and hymns... hence the diatonic fretboard and traditional basic tunings using the tonic note and a fifth for the most part (DAd, DAA). It's a little more challenging to play modern or complex music on a diatonic dulcimer than it is to play such music on a chromatic six string guitar. Some people who play various types of instruments will have different repertoires that they choose for a particular instrument, based on what that instrument is best at doing. There are no actual limits, but know that some things can be easier or more challenging!
If you play other instruments as well, you have a general musical jump start and advantage. But, you won't be able to play the same fingerings/chords you are used to between dulcimer and guitar or banjo.
