Well, he's not really an 'unknown', but there was an English singer/songwriter in the early 70's named Nick Drake who was virtually unknown in his own time and has only recently been getting much attention -- at least here in the US.

Though not really folk (certainly not old timey) he kinda fit that early 70's acoustic folk mold. He was an exceptional acoustic guitarist (he played with an incredibly fluid fingerstyle in odd alternate tunings) and had one of the most haunting voices I've ever heard. Some might find him a little too introspective and melancholy -- but if you like those qualities there no one I can think of much better. Five Leaves Left (Rykodisc) is a good introduction.

He died in 1974 of an overdose of anti-depressants. There's some question as to whether it was accidental or not.

Bryant

Five Leaves Left is a good introduction