Darling Jim. By Christian Moerk.
 
Two sisters and their aunt are found dead.  Investigators realize that the older woman had imprisoned and poisoned her nieces, and they had killed her in a doomed attempt to escape. 
 
A few weeks later, a package ends up the local post office’s dead letter bin.  It is undeliverable, addressed to “Anyone at all, Post Office, Malahide”.  A young artist/postman opens it, and finds the diary of one of the sisters.
 
The diary describes “Darling Jim” a charismatic storyteller and lady killer, who brought this doom down on the sisters and their aunt. 
 
This book is low on the reality scale – I wouldn’t recommend it to anyone who likes procedurals -  but great for atmospherics and local color. (It’s set in Ireland.)

Recommended.