The plot is fairly simple: the arrival of a child-vampire in a dreary, lifeless suburb of Stockholm, and the inevitable killings that follow, wreaks havoc among the residents. A group of drunks, a pious policeman, a sixteen year old who's embarking on a criminal career out of sheer boredom, and the protagonist Oskar and the boys who bully him all get involved. Lindqvist handles all their stories well, and there's never a quiet moment from beginning to end of the book. But I always felt quite distanced from the characters; it wasn't easy to really care about what happened to them.
I think that's because of the prose. Lindqvist writes in a dry, clinical style, faithfully recording every detail of the events following Eli's arrival in the suburb. There's something very methodical about the way he sets each event down, like a newspaper reporter determined to reconstruct the story perfectly and explain the character's motivations without really caring if the reader actually likes or feels sympathy for those characters. I don't think the fact that I read the book in translation helped at all; there were some scenes when I found myself wishing the translator had condescended to use a colloquialism every now and again, and others that just felt awkward. You could say that the book didn't "flow" very well, I suppose. Every now and then, you'd hit this little snag in the prose that annoyed you for some unknown reason.
Still, I enjoyed it very much. It was a fast and exciting read I'd definitely recommend if you have an interest in different interpretations of vampires. But there was that one little spark missing, for me at least, that would have made Let the Right One in truly frightening as well.