I got a little mixed up about whether the book or novel came first for this one, and having not seen the movie, this novel rang oddly hollow. It was very obviously initially intended as a film, and lacks the depth I'd expect in a novel.

I think ultimately the journal style/first person does it a huge disservice; it reads extremely young, as the character's in high school, but covers some serious topics. In a film, they'd be straightforward, but in a journal, they have an aura of absurdity to them - despite being very real and serious issues that teens face. Almost everything in the novel feels like a non sequitur. 

Having not seen the movie, I'd still say give this book a miss and watch that instead.