Once Dead, Twice Shy by Kim Harrison

Everyone does it. Dies, I mean. I found this out for myself on my seventeenth birthday when I was killed in a freak car accident on my prom night. But it was no accident. It was a carefully planned scything, just a small moment in the battle between light reapers and dark, heaven and hell, choice and fate. Only I didn't check out of my life like most dead people do. Thanks to a mistake, I'm stuck, dead on earth. The angel who failed to protect me and the amulet I stole from my killer are the only things keeping me from ending up where the dark reapers wanted me to be. Dead, that is.
My name is Madison Avery, and I'm here to tell you that there's more out there than you can see, hear, or touch. Because I'm seeing it, hearing it, touching it, living it. (vii)

Kim Harrison's Once Dead, Twice Shy doesn't waste time with an introduction. Seventeen-year-old Madison Avery now lives with her dad, who has no knowledge of the night his daughter died. She's being trained by a light reaper to try to extend her "life", with little success after four months. The killer is still trying to find Madison, finish the job, and reclaim his amulet to restore his full power. But when her body guard is called away and she finds herself under the watchful eye of an inexperienced guardian angel, Madison finds it necessary to tell the only other person who was there that night what really happened. On a race to save their lives, or in Madison's case restore it, Madison is left wondering who she can really trust.

AMY