This started out a bit slow, but once Matthew and Mark were really embroiled in the mystery at the monastery it was a fast read.
Matthew's reluctance to see the realities of the current religion was pretty frustrating. I suppose hindsight really is 20/20, especially centuries later. Because I am not (and never have been) a religious person, it's difficult for me to empathize with someone steadfastly sticking to their guns despite glaringly obvious clues to the contrary. Matthew's assistant Mark was set up to be the ~voice of reason~ during a turbulent time in the investigation, but I never really warmed to his character.
I did, however, like Brother Guy. I was hoping he wasn't the murderer.