The Suffering by Rin Chupeco
The darkness will find you.
Seventeen-year-old Tark knows what it is to be powerless. But Okiku changed that. A restless spirit who ended life as a victim and started death as an avenger, she's groomed Tark to destroy the wicked. But when darkness pulls them deep into Aokigahara, known as Japan's suicide forest, Okiku's justice becomes blurred, and Tark is the one who will pay the price...
I loved the first one, The Girl in the Well, so much I was ecstatic a sequel had been written. It's The Ring meets The Forest. The characters end up in Japan's Suicide Forest where the horror takes on a creepy and heart breaking dimension. Whereas the first book was written more from the ghost's point of view, this one is from the boy she possessed which allows the reader to understand what it's like to be the victim/willing participant of possession. The only problem I had was the emotional closeness that developed between him and the ghost and the fact that it would seem he was willing to give up the possibility of having a 'normal' relationship in order to fall for a ghost who essentially lives inside him.
*synopsis and pic from amazon.com