On the surface, Stolen is primarily a book about a kidnapping, which makes it a pretty appropriate title right off the bat. Still, the title applies to more than just Gemma's story—it's about Ty's experiences as well. The key is when he tells Gemma the story of being taken from his dad's farm to the orphanage and the trauma that resulted. "So they kind of stole you, too" (31.60), Gemma says, realizing that this guy isn't just her kidnapper but was once in kind of the same position she is now.
So, let's broaden our definition a bit. Stolen isn't just a book about kidnapping. It's a book about captivity in general and what happens when people are taken from their usual habitats and transported somewhere frightening and different. This is also why it's significant that the book is subtitled A Letter to My Captor. Gemma isn't absolving Ty of his actions—she recognizes him as the criminal that he is—but in choosing to explain herself to him specifically, it's possible she acknowledges their relationship and the similarities between them.