At first glance, it seems like it would be pretty obvious what's up with Gone Girl's title—it's a book about a girl who disappears, which means she's, well, gone. Duh. Maybe that's true, but there's more to Flynn's title choice than meets the eye—and in a lot of ways, it's actually describing Amy herself.
As we read Amy's diary entries throughout Part One, it's super hard not to like her. She runs the gamut from witty to downright hilarious. She likes "Summer. Birdies" and "Sunshine" (12.1); she uses phrases like "Let me set the scene" (8.1) and "Tra and la!" (2.1). Most of all, she just oozes comfort in her own skin and her marriage. "I like to think I'm confident and secure and mature enough to know Nick loves me without him constantly proving it" (8.12), she says. She's the kind of girl that men fawn over and ladies want to be besties with.
Hang on, though. Once the big twist is revealed—that Amy is actually alive and framing Nick for her murder—we learn the truth about Amy's personality. In reality, she's a sociopathic liar with a sick revenge streak. Diary Amy disappears from the picture entirely as we find out that she was totally fabricated.
When Amy decides to stop pretending to be a "cool girl" (30.23), dropping the act causes major strain in her marriage. "I hated Nick for being surprised when I became me," she says. "I hated him for not knowing it had to end, for truly believing he had married […] this figment of the imagination" (30.31). Um… Amy? You tricked him.
So Gone Girl is not just about a woman's literal disappearance—its title also addresses the identity crisis that erases the woman Nick thought he'd married and introduces him to the real Amy, who's not witty, not hilarious, and not cool, and who is actually horrifying beyond his—and our—wildest dreams.