Shadowland by Meg Cabot, writing as Jenny Carroll
This is the first book in "The Mediator" series, and I just flew through it!
Susannah has just been moved cross country by her mom, started a new school and has inherited three stepbrothers. All of this is fine...except that Susannah is a mediator - she can talk to, touch, and help ghosts.
Doesn't help that her new bedroom is inhabited by Jesse, a bodacious ghost who's seveal hundred years old. ;-)
This is such a fun and feisty read, and I can't wait to dive into more of the series!