Hi Jeffrey
Gregg Miner is affectionately referred to as the Harp Guitar Pope. He has made a study of the instrument and here is a link to the web site. I would recommend the CD “Beyond Six Strings” which is for sale on the web site. It is a nice introduction to a number of harp guitarists who perform on the CD. The site also lists a number of luthiers and lots of informative articles.
http://www.harpguitars.net/
If you are near Connecticut, you should come to the Harp Guitar Gathering this year. You will get a quick course on the harp guitar and be up to speed in no time. You will also see a lot of different types of harp guitars. Here is a link to the Gathering; they have one every year at a different location.
http://www.harpguitars.net/hgg/hgg.htm
You can also check out Steven Bennett’s web site. He has CDs and sheet music for about 100 of his songs, both 6 string and harp guitar.
http://harpguitar.com/