She'd known there would be ghosts in Oxford. Martha wasn't afraid of any headless horsemen, or nuns haunting the local ruins; it was Charlie, always Charlie she was afraid would find her.

When an anonymous letter is delivered to the Clarendon English Dictionary, it is rapidly clear that this is not the usual lexicographical enquiry. Instead, the letter hints at secrets and lies linked to a particular year. For Martha Thornhill, the new senior editor, the date can mean only one thing: the summer her brilliant older sister Charlie went missing.

After a decade abroad, Martha has returned home to the city whose ancient institutions have long defined her family. Have the ghosts she left behind her been waiting for her return?When more letters arrive, and Martha and her team pull apart the complex clues within them, the mystery becomes ever more insistent and troubling. It seems Charlie had been keeping a powerful secret, and someone is trying to lead the lexicographers towards the truth.

But other forces are no less desperate to keep it well and truly buried.