Haven't had a chance to look at this one yet but sounds interesting:
The Artist, the Philosopher, and the Warrior: Leonardo, Machiavelli, and Borgia: A Fateful Collusion
by Paul Strathern
Just bought paperback versions of two other interesting titles:
The Sistine Secrets: Michelangelo's Forbidden Messages in the Heart of the Vatican
by Blech and Doliner
Islamic Imperialism: A History by Efraim Karsh
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