As the Washington Post says:
A few years earlier, the monks made an amateurish recording of Gregorian chants; it went viral, exposing their existence. Ever since, tourists from around the world have been turning up at the abbey’s door, desperate to discover the source of spirituality that the chants convey. Every visitor has been rebuffed, till now. When Gamache and Beauvoir arrive, the abbey’s solid wooden door reluctantly opens because “their ticket was a dead man.” And not just any dead man: The victim discovered in the abbey’s walled garden with his skull bashed in turns out to be Brother Mathieu, the choirmaster who was the force behind the monks’ monster hit. The detectives find a scrap of vellum in Brother Mathieu’s fist with some “neumes,” or ancient musical notations, scrawled on it. Could this scrap come from a manuscript that would provide the long-sought-for solution to the “beautiful mystery” of the origins of Gregorian chant? Gamache and Beauvoir settle into spartan cells at the abbey to crack these and other even more vexing mysteries.