The ARG SIG Wiki of the International Game Developers Association looks like it could be a rich source to mine for info, especially their white papers...

I've read & "liked" several of the articles, but haven't noticed any change in the view counts and the [Sort By: Popular] filter doesn't seem to do anything. Am I missing something?

An example of an ARG like game played out in a 1997 movie starring Mike Douglas, Sean Penn, Deborah Kara Unger, et al. Could be watched & studied for various examples of ARG game play & story elements. Along these lines, I think there are probably lots of movies & books from which inspiration could be drawn for an ARG, e.g. genre, storyline or plot development, setting, etc. For example, it occurs to me that another one of my favorite films, Dark City, could also be great inspiration for an ARG with a bit of sci fi noir flavor to it: http://en.wikipedia.org/wiki/Dark_City_(1998_film)

This behind the scenes retrospective look at Perplex City ARG shows some of the elements that went into that game and might offer some interesting ideas....

Here's the Wikipedia entry for Perplex City which provides a helpful overview: http://en.wikipedia.org/wiki/Perplex_City

Another example of an AR geolocation based game. Also a great deal of outdoorsy fun! :-)

Introduction by Sean Stewart to The Beast ARG

Thought this content could be helpful from the standpoint of thinking about the psychological aspects of play and how they might apply to design & development of an ARG, as well as the playing of ARGs.

Thought this content could be helpful from the standpoint of thinking about the different types of immersion and their elements; and how these might be used in the design of an ARG.