I just ordered a book called "Three Ravens and Other Ballads", which is written specifically with accompaniment by lap harp. Sorry I don't know the publisher, I ordered it from Harp and Roses in B.C.I quite agree with the earlier "less is more" comment. Especially if you are singing older songs, leaving out the third is modally accurate and also gives you a nice, clean sound. I do use full arpeggios for variety, though, at whatever line of the melody I want to have more dramatic impact.
Cindy Ellen Morgan