The Courts of the Morning
Although not a Richard Hannay story as such (he narrates the introduction to the story), this is a wonderful story of revolution in a (fictitious) South American country, and features the cast of characters from the earlier Richard Hannay books - Archie, Sandy, Blenkiron, and the rest.
It also forms an important "back story" for the final book in the omnibus, "The Island of Sheep".