The Mediator by Meg Cabot

Suze is a mediator, someone who helps restless spirits cross over to the afterlife. When she moves to California, she discovers her new bedroom is haunted by the ghost of a 19th-century cowboy named Jesse de Silva—who’s handsome, mysterious, and very much not ready to move on.

As Suze balances high school life with ghostbusting, she encounters both dangerous spirits and personal dilemmas, including her growing feelings for Jesse.