The book starts out with a discussion about Belle Prater's disappearance, but the book isn't titled Belle Prater. Nope, it's called Belle Prater's Boy, because the true focus of the story is Woodrow Prater and how his arrival in Coal Station affects everyone around him, especially the narrator, Gypsy.
The word "Boy" in the title might also have a double-meaning and refer to Amos Leemaster, Belle's former beau. Even though Belle didn't end up with Amos in the end, his presence in Coal Station changed her life irrevocably. It's because of this "boy" arriving in town that Belle falls in love and eventually gets her heart broken. And this sets her on the very path that leads to her forming a family with Everett and Woodrow… for better or for worse.