[DocG]

Classical Gas (Mason Williams). A multipurpose tune - you can use it to annoy some people, impress others, or just to develop some useful fingerpicking chops.

Check out Lonnie Johnson, a monster player of old. He pretty much invented lead guitar, and has some great tunes. Especially those famous duets with Eddie Lang. I think he plays 12-string on those.

Before he got all blandified, Glen Campbell was known as a great 12-string player. His early stuff from the 60s is worth hearing, and inspired me to flatpick bluegrass tunes on the 12.

Then there's the legendary Dick Rosmini. Look for the non-overproduced sides, which influenced a lot of players.

And +1 to the suggestion of Gordon Lightfoot's "Sundown". Or anything else by him.