A coded letter. A missing woman. A truth waiting to be found. When an anonymous letter is delivered to the Clarendon English Dictionary office, it becomes rapidly clear that this is not the usual word-related enquiry. Instead, the letter hints at sinister events linked to a particular year. For editor Martha Thornhill, the date can mean only one thing: the summer her brilliant older sister Charlie went missing. Ten years on, Martha and her family are no closer to unravelling the mystery of Charlie's disappearance - until now. As more letters arrive, Martha and her team follow the linguistic clues to a troubling truth. It seems Charlie was keeping a powerful secret, and that someone is desperate to keep it well and truly buried. Guilty by Definition is a love letter not only to language but to the city of Oxford, wrapped within an intriguing mystery of a missing woman and considering the emotional aftershocks of her disappearance on those left behind.