Mystery: Murder on the Links by Agatha Christie

Hercule Poirot rushes to France in response to an urgent and cryptic plea from a client. But the Belgian detective arrives just too late: the man who had summoned him is found dead on a golf course, stabbed in the back with a letter opener and wearing an ill-fitting coat with a mysterious love letter in its pocket.
While the local authorities pursue the false leads suggested by the evidence, Poirot relies instead upon his famous "little grey cells" to cut through the confusion and untangle a story of blackmail, forbidden love, and a long-buried secret.