A charismatic but dangerous adversary stands between Richard Hannay and his mission to free the three hostages. The author also wrote "The Thirty-Nine Steps", "Greenmantle", "Mr Standfast" and "The Island of Sheep".
I enjoyed especially the respectful portrait of Hannay's wife, every bit as smart and tough as he -- quite surprising in an era (and culture) that I had assumed would be somewhat chauvinistic -- and a real relief from other spy stories in which the women simply scream helplessly until The Man comes along. Mind you, I have no political agenda -- and indeed am quite conservative about gender roles; but I just find it so much more sensible and realistic when women characters act like human beings!
"Hostages" is also remarkably prescient about the onset of WW2, and how Hitler would try to rule the world not merely through brute force but through propaganda and mass hysteria. There is also some fine thematic development here, esp. the notion that a spy mission may achieve "success" without "victory."
But the best thing about the book is its final chapter; as in "Standfast," "Hostages" has a split climax; the main conflict is resolved about 35 pages before the end of the book, and then there's a further, more nitty-gritty, down-to-earth duel at the end. Fantastic!
These books are great for folks looking for good old-fashioned adventure like James Bond, but without the girls and the violence.