The Rough Guides started out as a series of travel books for people on a budget.
They've expanded into other areas and produce an excellent book The Rough Guide to World Music which you may find useful in your search. In conjunction with the book they've produced a series of CDs featuring music from various areas of the world. Not all of it is 'folk' but they're a really good introduction, and I've found that each CD contains a couple of really cracking tracks.
There's an excellent site www.worldmusic.net which distributes the titles.