The deleted scenes got me interested enough in what else might be missing from the story to read the book.
Overall the screenwriter (also the book author) and director did a very good job condensing the plot to the bare essential which is the relationship between Eli and Oskar. There are a lot of subplots in the book which aren't really used in the movie, sometimes for the better.
First of all things are generally more bleak and Oskar is much more messed up in the book than he is in the movie. I guess they did it to make him more sympathetic but it helps explain why he'd be willing to run away with Eli at the end of the movie.
One thing we've discussed is whether Oskar will become the new Håkan, from reading the book this is impossible. There is no similarity in the relationship between Eli and Oskar and that of Eli and Håkan. While Håkan is indeed a pedo there is no sexual relationship between Eli and Håkan (though not for lack of Håkan desiring this, this is the draw to the relationship for him).
I'm not sure why they deleted the two scenes between Eli and Oskar now, perhaps to make the relationship more ambiguous? There is no such ambiguity in the book, Eli tells Oskar everything, and shows him after taking the shower. Oskar actually found out Eli used to be a boy before that, in the scene with the egg (which should be explained is a puzzle, and Eli is very interested in puzzles, which might help explain why the rubik's cube played such a prominent role in the movie, and also some of the connection they have at first) which led to a bit of soul searching that in retrospect I think would worked well in the movie but leaving it out gives the relationship a more innocent bent, considering their ages maybe that's not so bad.
The other big thing the book does is explain more about Eli, the author doesn't ever really explain how Eli became a vampire but does explain how he came to be castrated which Eli sort of tells/shows to Oskar. Eli is a much more tragic figure in the book and was fairly tragic in the movie to begin with.
They still leave the issue of just what the relationship is or will be up in the air, the end of the book is the same as the movie. The author does switch genders when talking about Eli (Elias) once he's shown Oskar that he's not a girl, or a boy really, but then he also has him pick out the dress instead of the clothing from Oskar's closet which was what was offered so I don't know... Anyway I've tried not to spoil too much, it's an interesting read but if you've seen the movie it will mostly fill in details and add in a few new characters.
Oh, one other thing that I thought was kind of interesting, the author started and ended his book the relationship between two Swedish outsiders with quotes from Morrissey songs. In fact the title derives from the second quote. Considering what a fan Moodysson is/was of Morrissey I wondered if maybe that was a bit of a shout out, though I guess it's not like the Smiths were a secret either.