The House on the Salt. Tide House Mysteries, #4

The pattern was never English. It only ever hid where no one would look. On the salt flats of the Languedoc, a schoolteacher drowns in a lagoon she has known all her life - a fortnight after she posts three careful pages to the police. Honor Whitling is nearly at the end of her career, and this case belongs to her successor, Maya Anders, who follows a fifty-year record kept beneath a salt-shed floor as the beloved foundation on the étang begins, at last, to give up its dead.
The fourth Tide House Mystery.