Patricia Wentworth 
Danger Calling [EPUB ebook] 

Lindsay Trevor was intrigued by Mr. Smith’s strange offer, but the fact that he was to be married the following Saturday led him to decline it. But he changed his mind when he suddenly received a letter from his fiancee cancelling the wedding plans. Soon he was plunged into a dangerous mission involving the fates of entire countries and the lives of eminent men, and Marian, his erstwhile fiancee, seemed to be mixed up in it, too.