Hmmm. You could raise a bit of controversy here, Paul. There are those here who might not consider Arlo Guthrie and John Prine to be "folk," although people here seem to be getting more tolerant lately. Let's see who I might recommend for someone with your tastes:
the late Steve Goodman and John Prine worked together on a lot of things. Steve wrote "City of New Orleans," which was recorded by Arlo. Steve did a lot of very creative (and very funny) stuff.
Tom Russell - many of his songs are about the Western U.S. He writes most of his own stuff, but there is a "timeless" quality to his songs. I think many of them will become classics.
Stan Rogers - I take it you're a Canadian. The late Stan Rogers is probably the most popular Canadian folk musician of the last 20 years, although there are many others who do wonderful stuff. Ian Tyson and his ex-wife Sylvia Tyson are also good examples. Another Canadian group I really like are Tamarack.
John McCutcheon is a master at most acoustic instruments, and he writes good songs for both children and adults. he also does a nice smattering of traditional songs.
Bill Staines sings mosly his owns songs, many about the western U.S. (although he's from New Hampshire). He writes the kind of songs that people love to sing along with.
For a gentle introduction to more traditional music, you might go to Folk-Legacy Records and order anything by The Golden Ring, Bok-Muir-Trickett, or Gordon Bok.
OK, so there's a start. I wonder what other people will recommend. There are so many good performers to choose from.