"Not that I shall, though," she added to herself, as she finished the letter; "and my dear aunt, if you do not tell me in an honourable manner, I shall certainly be reduced to tricks and stratagems to find it out."
There are no more uses of "stratagem" in the book.
Show samples from other sources
And I think of rats again and how sometimes, to pass the interminable hours, Teacup and I would plot our campaign against the vermin, stratagems and tactics, waves of attack, each more ridiculous than the last, until she dissolved into hysterical laughter, and I gave her the same speech I gave Zombie on the firing range, the same lesson that now comes home to me, the fear that binds killer to prey and the bullet connecting both as if by a silver cord.
Rick Yancey -- The Infinite Sea
If more is said that Marie-Laure cannot overhear—if there is a pantomime going on between them, notes passed, stratagems agreed upon—she cannot say.