I'm almost always going to give a five-star rating to book collections, simply for the fact that they offer more for the price of one. However, this collection of Richard Hannay novels is not just exceptional but also classic. Buchan invented the espionage genre with The Thirty-Nine Steps, which in my humble opinion has not aged well, not due to poor writing, but because it has been copied so much that the story beats have become spy novel tropes.
The 39 Steps begins with a regular dude getting involved in a mystery due to being gullible by inviting a woman back to his place. He then has to flee his home because he is suspected of being a contact for the beautiful (femme fatale) female spy. On her deathbed, she talks about her actual contact and avoiding the man with a missing finger. The story is basically paradigmatic of all spy novels, but that's not Buchan's fault.
Greenmantle further expands the spy genre by including code-speak. Then there is also a trip through the Middle East which might be read with an Atlas in-hand. I had an issue with the rather insensitive language, but I don't hold it against him because most writers of the time period (over 100 years ago) wrote using such unkind language.
The last book of the collection, Mr. Standfast, blends mystery novel elements with the newest tropes to the espionage genre, like going undercover as a pacifist, conscientious objector, Cornelius Brand. His contact is the lovely Mary Lamington, who he falls in love with and gets herself in danger. He must rescue her while continuing the mission and of course there's a ticking clock.
I disliked the constant exposition, but I really believe that is more a storytelling style that is outdated, and I think everything else was not just great, but perfect. Recognize that the inventors always get copied and in that copying, the story devices become cliche. I loved this collection.