I was not impressed with the Music Hound guide. It might be okay for someone just getting into things, but it's not comprehensive at all. There are many omissions and I find some of the descriptions to be a bit superficial. It is also focused on the Rounder/Flying Fish type stuff but neglects a lot of the ealier traditional music or topical songs. Still, it's better than nothing. No offense to Nightfall's opinion intended.
An excellent book to read is Baby Let Me Follow You Down by fingerpicker and illustrator Eric Von Schmidt. It is focused on the 1960s Cambridge scene, but that was one hell of a rich and varied scene. Lots of unbelievable photos and a good deal of interview material.
Harry Smith. Bob Dylan gave an interview in Rolling Stone a couple years ago saying that the role of the Harry Smith Anthology in the folk music revival is vastly overrated, it was just one of many sources. Personally, I found the Anthology interesting, but there's not much on there I want to hear more than once.
For a musical survey that's broader in scope and has catchier songs --check out the Smithsonian Folk Music set (or some similar title, four discs I think).
For a more narrow survey, but the best single collection of folk music there is IMO, check out Rhino's Greenwich Village Memories 3-disc box, which covers 1950 to 1970. Ranges from the very commercial groups to the more "authentic" ones, to the jug band and old-timey revival, to the birth of the singer-songwriter era. Great overview of the period - I'd expect you'll find a number of artists you want to hear more from on it.
All of the above has to do only with American Folk Music. International is a huge topic.