Meg Cabot - Ninth Key

The Mediator, book 02 - Ninth Key (2001) (writing as Jenny Carroll) aka High Stakes

Suze's new life in California is pretty cool. There's the pool parties, the new friends, and the fact that the hottest ghost in history happens to live in her bedroom. But when a screaming spirit appears at the end of her bed, Suze is thrown on to the trail of a murderer. All the clues lead to the freaky father of Tad Beaumont, the cutest boy in school... and the only guy who's ever asked Suze out. Not only is her potential beau's dad probably a killer but he also seems be some kind of vampire. So no one said that life as a mediator was going to be simple. But this is getting ridiculous...