Let the Right One In by John Adjvide Lindqvist

I'm going to try to keep most of the really big shocks under wraps. I'm going to have to reread this to make sure I'm separating the book and movie because it's been a minute and the plot's getting muddled.
After seeing this movie a couple years ago, I was floored. The sound quality, the subtlety and the acting made this movie really stand out to me. It was a little twisted, too; naturally I had to read the book as soon as possible.
If you strip away the vampire part, this book is a love story between 12 year old Oskar and his new neighbor, named Eli who he sees as a very strange girl to whom he's mysteriously drawn. Eli is Oskar's friend and confidante, the one he takes his problems to after being victimized by school bullies for years.
But you can't strip the vampire away. Eli isn't a normal little girl. This is obvious from the moment Oskar first encounters her on the playground. He comments on her smell, noticing that honestly, she smells like she hasn't bathed in years. Because she hasn't and she's been out of  practice being around normal people for so long, she's forgotten to clean herself. He sees her leap down from the top of the jungle gym, seemingly floating to the ground, he hands her a Rubik's cube that she returns completely solved the next day. And she begins talking to him by informing Oskar that they can't be friends, not ever.
While Oskar is trying to figure it out, he wonders if it's her very protective grandfather. Turns out, he isn't her grandfather. He's a pedophile who goes out and drains people's blood for Eli's sake in exchange for money. It's made clear, however, that he'd be perfectly willing to go without payment if he could only be intimate with Eli.
Oskar, meanwhile, has finally found a friend and an escape. He asks Eli to be his girlfriend. She doesn't say no, but her exact response isn't what it appears to be on the surface. She simply tells him, "I'm not a girl."
In spite of Eli's early warnings, they become very close. Eli becomes a protector and kind of mentor as well as a companion. She helps him fight back against the bullies who torment him. She is nearly discovered because she murdered one man, Jocke, by snapping his neck after draining him and a woman, Virginia, she's attracted the suspicion of a local man named Lacke who was friend to Jock and sometimes boyfriend to Virginia. Her guardian/pedophile is discovered while killing and willingly offers his blood for Eli. He falls from a hospital window, only to break open on the ground below. This doesn't stop him from becoming a vampire, however. Even undead, he still seeks out Eli. Virginia, in fact, also becomes a vampire and willingly commits suicide by exposing herself to sunlight while she's in the hospital.
The novel ends with Eli rescuing Oskar from the older brother of his main nemesis and Eli and Oskar leaving the city together, to meet what fate they may.