The House of the Schemers by Fred M. White

In any other circumstances Ailsa would have retired discreetly. But that strangely-addressed envelope under her very eyes fascinated her strongly. The man who until recently had been little more than a pleasant memory to her had suddenly become a strange reality. And only a few hours before Ailsa had heard that Ronald Braybrooke was dead.

But was he really dead? Or had John Stern purposely lied to her? Despite the questionable way in which Ailsa had made Stern's acquaintance, she could not bring herself to believe that he was wholly bad. Perhaps he had his own urgent reasons for concealing the truth; perhaps Ronald was not dead, after all. Anyway, that envelope pointed to the latter conclusion. But, on the other hand, it was possible that the writer had yet to learn what had happened in the North Sea.