Is there a console command to reset the courier so he resumes showing up and giving me "letter from a friend"? A day or so ago I could shout in any town, and then after a few fast travels or waiting he would show up. No longer. I do not have any letters on my characters and I do not have any unfinished "find the source of power..." quests.

Thanks.

After you use a dragon shout in a settlement, a courier will find you and give you a Letter from a Friend, which provides the location of a word wall when you read it.

Traveling to the location and learning the Word of Power from the wall completes the quest.

This quest can be repeated multiple times.

You could end the quest using: SetStage WICastMagic04 200

...and then try to shout to another settlement to become another "Letter from a Friend"

Locations: http://imageshack.us/f/401/4tbr.jpg/

This quest can be repeated multiple times (like Mark said), until you've learned all of the shouts in the game that aren't quest-locked. At this point, the courier will not come anymore, because locations with words of power that are quest locked, will never be given by the courier.