Shocker there. Mockingbird is about a girl who recently lost her brother in a middle school shooting. That's unfathomably traumatic, no matter who you are, and when Caitlin's school counselor and others are "dealing with" her often out-there behavior, I want to remind them that this is not just Caitlin being Caitlin. This is Caitlin dealing with something that would shake any of us to the core.
Katherine Erskine made a really interesting choice when she decided to tell a story about Asperger's syndrome together with a story about school violence. It lets us look at an angle at the grief and anger that are inherently part of the story, lets us think new thoughts about them as we see someone experience them who hasn't heard lots of discussion of similar incidents on the news. Kids in the book's middle-grade audience may be too young to quite remember the Virginia Tech shootings; this approach makes this kind of story new to the the person telling it. The focus on how Caitlin understands the world around her--both her brother's death and more ordinary moments--allows for some lightness in the telling, as well. (Really, though, someone should have thought to tell the literal Caitlin that "closure" doesn't mean "feeling completely better.")
I'm well into Francisco Stork's The Last Summer of the Death Warriors now--another fabulous book that also begins with the loss of a sibling and then introduces more difficulties. There may need to be some happy reads in the near future.