The Absent Heart is very much a novel about letters and how they defined Frances Sitwell’s relationship with Robert Louis Stevenson, but it’s not a novel of letters. I’m actually quite a fan of the epistolary novel and, with all of the R.L.S.’s letters to my heroine at my disposal, I could have gone down that road. But these letters are published and have been for many years. They are well-known, if only by the cognoscenti, and even if I had sought permission to use them (the main edition i...