Number45 said:
Do it do it do it - one of my favourite books ever. I haven't seen the film yet, and frankly if it's as evil as the book I'm not sure I want to!
Well, that certainly makes the decision easier, then.
I would definitely recommend you to watch the original film adaptation (in Swedish), "Låt den rätte komma in". (I can't speak for the American adaptation, though, but it's supposedly decent too.)
I personally loved it, and while everything wasn't explicitly explained, the things implied were definitely very dark. The author also wrote the screenplay, too. I heard that early disc-based releases of the film had extremely simplified subtitles, but I think that's fixed (and something I didn't notice due to understanding the language).
Such a unique take on the medium of horror, dark romance, and vampires. I generally don't care for Swedish cinema, but this one surprised me.