The Girl from the Well by Rin Chupeco
I am where dead children go.
Okiku is a lonely soul. She has wandered the world for centuries, freeing the spirits of the murdered-dead. Once a victim herself, she now takes the lives of killers with the vengeance they're due. But releasing innocent ghosts from their ethereal tethers does not bring Okiku peace. Still she drifts on.
Such is her existence, until she meets Tark. Evil writhes beneath the moody teen's skin, trapped by a series of intricate tattoos. While his neighbors fear him, Okiku knows the boy is not a monster. Tark needs to be freed from the malevolence that clings to him. There's just one problem: if the demon dies, so does its host.
If you liked the horror movie franchise The Ring you'll love the book. It definitely has a great horror quality to it but best of all it kept me riveted because I could not put it down until I knew how it was going to end. The only disappointing thing about it was that it did eventually end. First time I ever rooted for the "evil" character.
*synopsis and pic from amazon.com