The Book of the Kosst Amojan was an ancient Bajoran religious text which dealt with forbidden dark knowledge of the Pah-wraiths. It was considered highly dangerous and held in the Archives with no one allowed access to it except the Kai. The book contained information on how to release Pah-wraiths from their prison within the Fire Caves of Bajor.
Winn Adami and Skrain Dukat later brought the Book to the Fire Caves where Winn read the incantations. They succeeded in summoning the Pah-wraith, although Winn was shocked when they chose Dukat as their Emissary. As Dukat and Benjamin Sisko began their final clash, Winn tried to destroy the Book. She was later killed by Dukat, but not before drawing Sisko's attention to the Book. Seizing his chance, Sisko threw himself at Dukat, knocking himself and his nemesis into the fires. Dukat held onto the Book, even as it was destroyed by the flames.
After being transported to the Celestial Temple, Sisko was told by his mother Sarah that the Book of the Kosst Amojan was the key to releasing the Pah-wraith that could never be used again. (DS9 episode & novelization: What You Leave Behind)