The Dark Vault — V.E. Schwab

Each body has a story to tell, a life seen in pictures only Librarians can read. The dead are called Histories, and the vast realm in which they rest is the Archive. Mackenzie Bishop's grandfather first brought her here four years ago, when she was twelve years old, frightened but determined to prove herself. Now her grandfather is dead, and Mac has grown into what he once was: a ruthless Keeper, tasked with stopping often-violent Histories from waking up and getting out. Because of her job, she lies to the people she loves, and she knows fear for what it is: a useful tool for staying alive.