I don't think I can overstate Fahey's importance to American folk music, as much as Fahey himself hated the restrictions of the genre. Fahey was a fingerstyle guitarist, which meant that he was championing a somewhat archaic way of playing guitar when he first started recording himself in the late 50s. Fingerstyle means exactly what it says: the guitarist picks notes on the guitar with fingers rather than a synthetic pick. Although Fahey's style has been called American Primitivism, there's nothing primitive about his style or that of the guitarists who he nurtured (well, he wasn't the nurturing type, so let's call it "supported" instead) and influenced. Their flurry of sounds, usually employing at least two voices on the guitar (a bass line and a melody line), but often adding a third voice (usually a mid-range drone), is anything but primitive. Fahey had one ear in the past, always looking for old blues guitarists to learn from, but his other ear could only hear the future, and his uncompromising avant-garde tendencies give his instrumentals a timeless quality.
Christmas Guitar (1968 - 1986). This compilation includes most of the wonderful 1968 album The New Possibility: John Fahey's Guitar Soli Christmas Album, plus a bunch of other Christmas tracks that Fahey recorded over the next 20 years. I reviewed it a little over a month ago.
You really must read the liner notes to get a glimpse of what working with Fahey was like. I do not envy Glenn Jones his experience of having his idol sabotage his plans, but the result is pretty great in spots and a fascinating trainwreck in others.