Richard Harding Davis 
The Spy [PDF ebook] 

Mr. Crosby is a high-ranking official known to be often entrusted with secret government missions. But when he finds himself in Valencia, that's entirely an accident and has nothing to do with any such mission at all. But his reputation does not allow him to simply leave a region riddled with conflicts, and so, a spy attaches himself to Crosby. On a small boat with only one another for company, things can quickly get complicated.