Most of the paper assumes that the recommender creators will allow the user in on the thinking process that is going on. They performed an experiment in which the recommendations were presented in 21 different manners. Some gave the "supposed" reasoning behind the recommedation while others just presented the recommendation in a different format. The best performing presentation was a simple histogram with 3 values(good, bad, neutral). This binary choice beat out a 5 bar histogram (1 - 5 stars). That was unexpected to me. Unfortunately, the link to all 21 representations that were used in the experiment no longer works. The paper is reasonably easy to read and interesting.
The application recommender was for movies (movielens). The website is: http://www.movielens.org/login
"Explaining Collaborative Filtering Recommendations", Herlocker, Konstan, Reidl, 2000 ACM
http://www.grouplens.org/papers/pdf/explain-CSCW.pdf