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 room, 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.
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.
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.