Guilty by Definition by Susie Dent

Martha Thornhill comes back to her hometown of Oxford to take a job as the senior editor of the Clarendon English Dictionary (think OED). She spent the last ten years living in Berlin, trying to put the disappearance of her older sister Charlie behind her. When letters start arriving addressed to Martha and her colleagues Simon, Alex, and Safi that seem like coded messages related to Charlie, the quartet can't ignore the clues in front of them. The police become involved again in the investigation, and the dictionary group start following their instincts and interviewing people themselves. It seems that maybe Martha and her father Gabriel will finally have closure. Guilty by Definition is a leisurely-paced novel for those who enjoy etymology, puzzles, and mysteries set in academia.