This is the homepage of the Prairie Home Companion site. There, you'll find a lot of information about the history of Keillor's radio show, a photo, and some links to related articles.
This page from the archives of the New York Times contains links to reviews of some of Keillor's work and to other articles about the author. (Free registration required.)
Maybe you're interested in researching humor, but not sure how to narrow your topic. This directory from Yahoo.com should give you plenty of ideas about how to proceed with your research.
Keillor himself examines the cultural impact of the fictional town he created in this National Geographicpiece entitled “In Search of Lake Wobegon.” What you'll find online is an excerpt. What's an efficient way of tracking done the whole essay?