Categorical Clustering of Users Reading Habits
Sounds like you are set to do collaborative filtering - you have readers (users) and documents (items) creating a unary-response matrix with 1 cells indicating the reader (row) read the given document (col). There are various types of recommender systems (e.g., user-based and item-based collaborative filters) and several of them can account for the meta-data associated with your documents (e.g., content-based systems). You should look into R package {recommenderlab}