Buchan’s spy series (begun with The Thirty-Nine Steps) continues into WWI with Greenmantle and Mr. Standfast, which I think is the best of the three. Our hero, Richard Hannay, is called up for another secret mission. Initially hesitant to leave command of his troops on the Western Front, Hannay is convinced by his superior that the war will be lost should he fail to accept this do-or-die mission to track down a German spy.
Hannay’s first undercover assignment, as a pacifist living in an artist colony, is amusing as the patriotic solider battles with the antithetical role he’s forced to play. In the second half of the book, after a chase around the British Isles and a succession of disguises, Hannay is back in uniform. The book concludes with harrowing front-line fighting and an exciting aerial dogfight.