The Book of Secrets - The Last Oracle, #1

Helena Davies just wants a job that will get her out of her parents' basement, but by the end of her first day at Abernathy's Bookstore, she has a dead boss in the basement and the news that she is now a part of an endless magical war. Abernathy's is the world's only living oracle, and Helena is now its custodian. Without any training, she must navigate her new world and find a place for herself within it.
But there's still a murderer on the loose-and Helena might be next on his list.