Twilight (The Mediator Book 6) by Meg Cabot

What's on the back of the book:
A girl. A guy.
A new kind of ghost story.

You want me to make your boyfriend disappear?" Paul asked.
His body was warm against mine, so there was no other explanation for why my heart went suddenly cold as ice, except that his words terrified me to the point that my blood seemed to freeze in my veins.
"We have an agreement," I said, my tongue and lips forming the words with difficulty because they, like my heart, had gone ice-cold with dread.
"I promised I wouldn't kill him," Paul said. "I didn't say anything about keeping him from dying in the first place."

A piece from the book:
This time it's life or death

Suze has gotten used to ghosts. They wake her up in the middle of the night. They haunt her locker at school. She's even spotted a few down at Carmel Beach. Suze is a mediator, after all, and communicating with the dead is all in a day's work for her. The last thing she ever expected was to fall in love with one: Jesse, a nineteenth-century hottie.
But when she and Paul Slater, himself a mediator of undeniable power (and dubious intent), discover that the powers they share aren't limited to helping ghosts resolve their earthly woes, but can also be used to determine whether or not they become ghosts in the first place, Suze can't help but freak. Not because she suddenly knows how to alter the course of history, but because Paul can, too. And Paul would like nothing better than to prevent Jesse's murder, keeping him from becoming a ghost and allowing him to live a natural life at last . . . but in the nineteenth century. Meaning Jesse and Suze would never meet.
Suddenly, Suze is faced with the most important decision of her life: allow the only guy she's ever loved to have the life he's always longed for . . . or keep him anchored forever in half-life at her side. Will Jesse choose to live without her, or die to love her?