At the End of the River Styx

Before he can be reborn, Zan has spent 499 years bound in a 500-year curse to process souls for the monstrous Ferryman—and if he fails he dies.
In Portland, Bastian is grieving. He survived a car accident that took his mother and impulse-purchased a crumbling bookstore with the life insurance money.
But in sleep, death's mark keeps dragging Bastian into Zan's office. It shouldn't be a problem to log his soul and forget he ever existed. But when Zan follows Bastian through his memories of grief and hope, Zan realizes that he is not ready for Bastian to die.
The boys borrow time hiding in the memories of the dead while the Ferryman hunts them, and Zan must decide if he's willing to give up his chance at life to save Bastian—and Bastian must decide if he's willing to keep living if it means losing Zan.