At the end of this richly complicated medieval mystery book, which is also so much more than that, the murderer is clearly identified as being Brother Jorge, who killed the brothers in the abbey to prevent them from finding and spreading the news that Aristotle's Second Work, which Jorge considered was so dangerously subversive because of its message on humour and laughter. Note how Jorge admits to the murder of Abo:

I could no longer trust him. He was frightened. He had become famous because at Fossanova he managed to get a body down some circular stairs. Undeserved glory. Now he is dead because he was unable to climb his own stairway.

William manages to uncover the murderer at the end of this work, though unfortunately he is in many ways too late to bring Jorge to justice, as Jorge kills himself by eating some of the poison-coated pages of the book he did everything to keep from others, and then running into the secret library and setting fire to it. The end result is that not only the library and all of its books but also the entire Abbey is destroyed and ruined. Note how the "Last Page" makes this very clear:

The Abbey burned for three days and three nights, and the last efforts were of no avail. As early as that morning of the seventh day of our sojourn in that place, when the survivvors were fully aware that no building could be saved, when the finest constructions showed only their ruined outer walls...

The fire therefore that destroyed the library ended up destroying the entire abbey, and ending the work that was done there. Given Jorge's obsession with the end times and the Book of Revelation, and the way in which others read the sounding of the trumpets into each of the murders, it is perhaps fitting that the final seventh trumpet, which was due to herald destruction and chaos, resulted in the destruction of the entire abbey. It is a fitting climax to the novel and also a potent message concerning the danger of literature, that somebody would be willing to destroy not only the book itself but indeed the whole structure in which it was housed.