The Harlequin Opal (Vol. 1-3). Fergus Hume

Four school friends arrange an appointment in 15 years and are committed to keeping it, no matter what surprises and unforeseen adventures are prepared for them by life. A novel starts with the reflections of one of the protagonists on whether the others will keep the promise or he is the only one to keep the foolish promise of childhood. All four come. Father Time has given each of them a peculiar role in life. One is a respected doctor. The other one is an archeologist. The third one is a correspondent. The fourth hero is a traveler. Neither of them is married, and that means they are free to depart from the monotonous routine of their loves into an exciting journey full of mysteries, betrayal, love, politics, and adventures that will keep the reader suspended from the first to the last page of the book.