The sentences are not so long, they are pretty easy to understand.She is telling quite a lot about the persons in the book but more about their personalities. There is a lot of dialoges both from the emails and in the real world. When you read the book you can notice that Jamie/Jamilah is telling from her perspective. There are a bit difficult words but some of them you do understand because it's explained by the sentence.
Everything is connected to her just because she is the narrator so pretty much everything is told by her.
It is, definitely! Which also means that it is proper for the language to be less complex than if the story hade been told by an adult. Obviously, the e-mails also add to the more informal language, yes. Good!
SvaraRadera