S5·E7 From Doon with Death: Part One

Margaret Parsons, a fairly ordinary housewife who, with her water board official husband Ron, has recently moved to Kingsmarkham, is found murdered in a field. Mrs. Parsons led an extremely uneventful life, being a lay preacher, but Inspector Wexford is intrigued when he is looking through her belongings and fine a number of expensive antique books all inscribed 'From Doon to Minna'.Who is Doon?