The name of the rose. The movies...fine, I guess. The books at least 300 pages too long and frequently segues into long-winded discussion of the political minutiae of the warring monastic orders during the reign of Pope John XXII.
If you want to read about the time period you'll be annoyed by the murder mystery shoehorned into your dry long winded historical fiction. If you wanted a murder mystery set in a historical setting then you'll be annoyed by the history lesson being shoved down your throat like a dehydrated fig newton.