Along with Greenmantle, I'd place The Three Hostages and The Island of Sheep as John Buchan's best "Hannay" books.
The Three Hostages takes place in 1921 when (now) Sir Richard Hannay is about 44. He is approached by the Home Office to assist when the loved ones of 3 prominent political and business figures are kidnapped by a nefarious local businessman who is touted to become an important political figure in the future of mother England. Hannay is tasked with working undercover discovering the whereabouts of the hostages while Scotland Yard investigate the abductions officially. Hannay is their 'secret weapon' and he enlists the aid of a few old faces from previous stories and even his wife in the course of the story. The writer weaves an intricate and suspenseful tale and I found the book hard to put down. It's a long while since I read a book that -figuratively at least- had me on the edge of my seat. The denouement didn't disappoint either, but like all good books, I was sad when it ended. This is writing at it's finest.
The Island of Sheep sees Hannay called by an old oath to protect the son of a man he knew when a young man in South Africa before WWI. This chap apparently safeguards the secret of what is referred to as "the greatest treasure on earth". Young Peter John, Hannay's son is also focussed in this story. He is now a bright, if serious teenager, and a falconer of some talent, who finds himself abducted and used as leverage by the syndicate of ne'er do well's hunting for this 'great treasure'.
This one is another page turner. Didn't quite have the suspense of The Three Hostages, although it's still a well written book and a fun read.
I love these 'old' stories for the insight they give to how people lived thought of life in the early 20th century. I like to compare my impressions of life then with life in the early 21st century and how much things have changed....or not. Politics certainly seems to be politics regardless of the century!