We will be discussing "No Night Is Too Long" by Barbara Vine (aka Ruth Rendell).
(NB. There is a Kindle edition but it's quite expensive and you'll have to search for it separately as the entries aren't linked.)
Tim Cornish thought he'd gotten away with murder. For months after he'd killed his lover off the Alaskan coast, there hadn't been a word about the murder. But then the letters started to arrive, giving intimate details of the murder. It seems that someone knows what Tim has done.