This book is basically a darker more teen-y version of the Disney TV movie Susie Q from the 90s. They share the same premise- teenage dead girl's ghost meets handsome teenage boy who's the only one who can see her, teenage boy helps her remember who she was, and find her family. Plus a little touch of young adult paranormal romance.
The dark elements in this book stem from a mysterious evil ghost, Eli, who "wants" Amelia to be his apprentice/evil side kick. Eli comes from some dark netherworld, hell pit, or whatever it is underneath the bridge where Susie, I mean Amelia, fell to her death. I didn't really understand that part, but this was the first in a series so maybe it gets clearer in the next book.
This book didn't really pull it off for me. The writing is kind of boring, the bad guys not really well explained, and all together just missing something to truly make it worth reading. If the story intrigues you- go find an old copy of Susie Q. It's funnier, cuter, much more entertaining, and has a better ending.