I've been away, so I'm seeing this for the first time, sorry. My (many) suggestions:

Lord Chesterfield: Letters
Michel de Montaigne: Essays
Isaiah Berlin: "The Hedgehog and the Fox"
Daniel Kahneman: "Thinking Fast and Slow"
John Keegan: "The Mask of Command"
Niccolo Machiavelli: "The Discourses"; "The Prince"
Sun Zi: "The Art of War"
Helmut von Moltke: "The Art of War, selected writings"
Lee Kuan Yew: "The Singapore Story"; "From Third World To First"
Nassim Taleb: "Fooled By Randomness"
Epicurus: Writings
Aristotle: The Art of Rhetoric
Eric Hoffer: The True Believer