Things Not Seen is a pretty self-explanatory title for a story about a boy who becomes invisible since Bobby Phillips literally can't be seen once he wakes up with his condition. He can't see himself, and neither can anyone else; he starts moving through the world like some kind of ghost.
But Bobby's body isn't the only thing that isn't seen. The title also refers to all the invisible yet super important things that hold people together. We're talking love, friendship, and loyalty. Even though you can't see Bobby's parents' love for their son, or Alicia's friendship and support, you can feel them. They can't be proven or viewed by the naked eye, but there's nothing more important than these threads that bring the characters together.