Let the Right One In by John Ajvide Lindqvist

It is autumn 1981 when inconceivable horror comes to Blackeberg, a suburb in Sweden. The body of a teenager is found, emptied of blood, the murder rumored to be part of a ritual killing. Twelve-year-old Oskar is personally hoping that revenge has come at long last—revenge for the bullying he endures at school, day after day.

But the murder is not the most important thing on his mind. A new girl has moved in next door—a girl who has never seen a Rubik’s Cube before, but who can solve it at once. There is something wrong with her, though, something odd. And she only comes out at night. . .