Dean... Every notebook on this particular shelf tells a version of how you die. You specifically, heart attack, burned by a red-haired witch, stabbed by a ghoul in a graveyard, and on and on... But which one’s right? That depends on you, on the choices you make.
Existing on an unknown plane of existence, this library, which the new incarnation of Death, Billie, refers to as her reading room, houses a near-infinite library of notebooks, each of which lays out in detail a possible death for every being on the planet. Deaths are selected through the choices a person makes in life. When the fate of one person changes or doesn't come to pass, the notebooks rewrite themselves.
All of the books in the library write themselves and can only be viewed by someone other than the current incarnation of Death if Death allows it. Not even God can access the books. However, God has a book in the library that details God's ultimate demise.
When Billie needs to have a private conversation with Dean about his recent visit to an alternate universe, she shows Dean his notebooks and lists off a few of the possible deaths he may end up having, telling him he may have a heart attack or be burned alive by a red-haired witch or simply be stabbed by a ghoul in a graveyard. She tells him that any of these could be the way he dies depending on his choices but Dean's current suicide is not listed and she sends him back.
Jessica tells Sam and Dean that Death keeps notebooks on all the ways a person may die depending on their choices. Dean says he saw his, that he has a whole shelf. In Rowena's notebook, her death is always the same: Rowena's real permanent death is at the hands of Sam Winchester.
Later, when Rowena remarks that Sam is going to be the one who kills her, Sam says that it might be possible to change her fate, just as she changed the fates of others.
Billie visits Dean in his bedroom in the Bunker after Michael has been locked away in Dean's mind. Billie reminds Dean of his trip to her reading room and tells him that all of the notebooks containing Dean's fate have changed; all but one now show Michael escaping and using Dean's body to destroy the world. Billie hands Dean the notebook containing the one fate where Michael loses, shocking him with what it reveals must be done.
Rowena decides to use a spell from the Book of the Damned that can absorb the demons and hell ghosts and imprison them all, along with herself, in Hell. The spell requires her still-coursing blood and her final breath, meaning that in order to commence the spell, she has to be mortally wounded. Rowena asks Sam to do it, saying that she can't do it herself and reminding him that her final demise is in Death's books. Sam is reluctant, saying "screw the books", but Rowena says it's the only way to save the world. With no other choice, Sam mortally stabs Rowena, commencing the spell. As she walks towards the rupture, all the souls of Hell trapped by Belphegor's spell are absorbed into her wound. Rowena says goodbye to her friends as she falls into the rupture just as it is about to close, fulfilling the prophecy of her mortal demise.
When Castiel calls on Sergei to help with Sam's wound, he instead puts Sam's life in danger. Sergei says he will only heal Sam if Castiel and Eileen find the Black Key that will open a door to Death's Library and give it to him. Castiel has, however, arranged for Bobby Singer to find Sergei's niece, and he threatens to have her harmed if Sergei does not heal Sam.
Billie reveals to the Winchesters, Castiel and Jack that even God has a book in Death's library that details God's ultimate demise. Billie explains the nature of the books and will only state that the book shows that Jack is responsible for God's ultimate demise.