Wow, I suck. This is a very belated response, and I apologize for my slothfulness. I have been meaning to compose this entry for some time, but only now am I actually getting around to it. Since I posted my first response, I have been able to read the book and gleam a few insights from it. That being said, I'll get on to my response.
Book vs. Film
I really enjoyed the book. The movie created a strong desire to know more about the characters, especially Eli and her background, and the book helps fill in some of the back story. The book also contains more of your conventional horror elements. It has more dark scenes, more disturbing elements to the story than the film, and as such, is quite separate from the film. What director Tomas Alfredson and novelist and screenwriter John Ajvide Lindqvist were able to do was extract the most important elements of the novel, primarily the relationship between Oskar and Eli, and create a focused and artistic film that stands entirely on its own. In fact, I would recommend that anyone interested in seeing the film and reading the novel go about it the same way I did: backwards -- film first, novel second. I feel the film achieves a greater artistic unity than the novel. There is really nothing I would change about the film if I could somehow go back and alter it during production.
The Title
Personally, I love the title Let the Right One In. It has thematic importance and alludes to vampire mythology and the requirement that vampires be invited into a room before they can enter. Several times Eli asks Oskar if she can come in, and the scene where Oskar taunts her and forces her to show him what happens if she enters uninvited is pretty astounding. The title factors significantly into the relationships of the film. Both Oskar and Eli are very lonely, and both need to "let the right one in" to their lives, to move beyond their solitude and enter a better, healthier way of life. There is a section in the novel that further explains the significance of the title. It comes after Lacke, despondent upon learning of Jocke's fate, tells Virginia, the woman he loves, that he has no one now that Jocke is gone. Virginia storms away, hurt and angry, and has a nasty surprise in store for her. Before she receives that surprise, the novel gives us insight into her thoughts:
Let a person in and he hurts you. There was a reason why she kept her relationships brief. Don't let them in. Once they're inside they have more potential to hurt you. Comfort yourself. You can live with the anguish as long as it only involves yourself. As long as there is no hope.
While that explanation comes from Virginia, it's most applicable to the relationship of Oskar and Eli, which I will further analyze in the next section.
Oskar and Eli
I feel like posting a few more thoughts, some informed by the novel, about the story's central relationship. The actors cast in the roles of Oskar and Eli were phenomenal, though I learned that they dubbed a less feminine voice over the actual actress who portrays Eli. Apparently she sounded too much like a girl. Anyway, one of the aspects of the story that has most intrigued me and remained in my thoughts is the co-dependence that develops between Oskar and Eli.
Oskar, more than anything, needs a friend in his life. The movie portrays him as completely friendless; he is picked on by bullies at school and plays alone in the courtyard outside his apartment building after school. Only the gym teacher shows Oskar a little sympathy, but that is not enough for a twelve year old child. Because of his alienation from his classmates, Oskar develops unnatural and violent tendencies, and this only alienates him more. He has a few nice moments with his parents -- I love the scene where he and his mom are brushing their teeth, and I was pretty moved by the joy he felt at times when he was visiting his father -- but those moments don't last. Oskar's family is broken, and his parents cannot understand him. He is in desperate need of someone, anyone to offer a sympathetic smile and an understanding heart. Eli fills that void in his life, and for the first time in Oskar's life, he has a friend. (The novel gives Oskar a few friends, but they are not around when he needs them and cannot be the type of genuine friend he needs in his life). Oskar comes to depend on Eli's presence in his life. An image from the film really drives that point home for me. The morning after Eli leaves town in the taxi, Oskar looks out his bedroom window at the empty courtyard. The shot is framed with the camera outside the window, looking in at Oskar. He has clearly been crying for some time, the pain caused by Eli's absence being too much for him to bear.
What strikes me, and this might just be me once again reading too much into things, is that Oskar looks like Eli when she needs blood. He has a similar paleness and red, sunken eyes. Like Eli needs blood to survive physically, Oskar needs Eli to survive emotionally. He is dependent on her.
The film portrays Eli in a slightly different light than the book, and I think the differences are noteworthy, especially when looking at her relationship with Oskar and whether it is genuine and sincere or merely masterful manipulation to ensure her continued survival. In the book, Eli is a clumsy and ineffectual killer. She has a number of blunders and is clearly in need of a helper. Enter Hakan and later Oskar. In the film, however, she is portrayed as a better killer. What sets the film portrayal apart is that Eli seeks assistance, not because she is a blundering vampire, but because she doesn't enjoy killing. After Hakan messes up in the beginning of the film, Eli angrily asks, "Do I really have to take care of this myself?" An ineffectual killer would act more frightened than angry in this situation, but the film shows Eli very angry in this scene.
The film leads me to believe that Eli seeks Oskar's companionship out of a genuine need for friendship, not because she needs another assistant to fill Hakan's position. The book clarifies the relationship between Eli and Hakan, and while I don't want to go into particulars, I will say that it is very, very different from the relationship between Eli and Oskar. Though, like you pointed out, Justin, Oskar will very likely end up in Hakan's position later in life. Another element of the story, this one from the novel, that leads me to view their relationship as genuine, especially on Eli's end, occurs as she is able to help Oskar literally see things from her perspective:
For a few seconds Oskar saw through Eli's eyes. And what he saw was...himself. Only much better, more handsome, stronger than what he thought of himself. Seen with love.
I love that passage and think it really clarifies a lot of the uncertainty regarding their relationship. The film never gives such an explicit defense of the genuineness of their relationship, but I think it can be inferred from the way Oskar and Eli look at each Other. Eli depends on Oskar's friendship more than she will depend on his assistance to acquire blood. She is more than willing to leave in the dead of night and start over somewhere, and that willingness tells me that she is capable of living on her own and probably has at some point during her very long existence as a vampire. She chooses Oskar's companionship, not to avoid killing for herself, but to have love and friendship in her life, for however long it might last.
Anyway, I have written more than enough for now, though I'm sure there is a near endless amount of things to talk about regarding this film. It is definitely one of the best films of 2008, and I'm doing my best to share it with other people.