Clements is so real-to-life. He knows how a kid feels. He tells most of this story through the main character's thoughts, and he gets all the emotion and thought processes spot on while incorporating good, healthy morals in the character development we can easily see happening on each page.
One of the main reasons it's so relatable is just that Jordan is so... well, average. There's nothing outstanding about her. There's nothing outstanding about her thoughts (although there were so many times I wanted to cheer for her good decisions). She's the regular everyday kid walking the halls who doesn't make a splash one way or another. And yet she's a hero in so many ordinary ways.