Lake Conshe

Lake Conshe, by Ingrid Lynch, is not a whodunit; it is always obvious who the murderer is. But the book does look at how war and personal tragedy can affect a person...
The Stedman/Robbins family owns the cottage on Lake Conshe. Helen Stedman is of noble birth and has funded the foundry her husband Reginald runs, along with his manager Gunnar Thorson. Their son (Stedman Robbins) and Dalt Thorson are the best of friends; Dalt becomes Helen's ward after his parents die from pneumonia.
The family moves to Pennsylvania in 1935, and the boys, firm friends by now, go off to school together. Trouble comes for Sted, though, after the boys graduate. He wants to marry his sweetheart, Faye but is told he must go to college or be cut off with no money. Dalt is to stay and look after Helen, whose health is failing. Sted goes to college, and while there Faye is killed in a traffic accident. An awful argument ensues, and then he goes off to war. Dalt again remains behind to look after Helen.
Fast forward to the 1980s. Don Axel (head of police) is suspicious of Sted as people who have all visited The Cottage are going missing. He wants to investigate Sted and search the property but is blocked from doing this due to the good standing Sted has in the community and the vast amounts of money he donates.