Since this is a story about a boy who becomes invisible one day, it would make sense if the book ended in the moment when he returned to being visible, right? Not so fast, though. The most important aspect of Things Not Seen isn't Bobby's weird condition; it is the way that he grows to know himself and learns how to love over the course of the book. That's the real journey, and it doesn't end just because he can suddenly see his legs. Because of this, the book ends when he's going to reveal his feelings to Alicia:
I fold up the letter and stick it in my pocket. As I open the front door, something moves. I turn, startled. It's me, in the big hall mirror. But I don't stop to look. I'm out the door. I'm in a hurry. I've got to get over to Alicia's house. I need to tell her how much I love… how much I love her poem.
And I need to be there to see her face when I tell her. (28.97-98)
In this moment, Bobby doesn't need to look into the mirror to know who he is. He knows himself now, and since he's a boy in love, he's going to do everything he can to keep Alicia in his life.