Guilty by definition

"When an anonymous letter is delivered to the Clarendon English Dictionary, it puzzles the team of lexicographers working there. It soon becomes clear that this is not the usual eccentric enquiry. The letter hints at secrets, lies, and a particular year. For Martha Thornhill, the new Senior Editor, the date can mean only one the summer her brilliant, beautiful older sister Charlie went missing. After a decade spent living abroad, Martha has returned to her father, her home, and the city whose institutions have defined her family. But the ghosts she had thought to be at rest seem to have been waiting for her to return. When more letters arrive and the team pulls apart the clues within them, the questions become more insistent and troubling. Charlie had been keeping a powerful secret, but as the mystery of her disappearance starts to unravel, someone is trying to lead the lexicographers to the truth, while another is desperate to keep it buried"--