I nominate
The Man Who Was Thursday by
GK Chesterton.
Amazon quotes a critic who wrote about it: "Chesterton's tour de force is a thriller that is best read slowly, so as to savor his highly anarchic take on anarchy."
The Man Who Was Thursday is availalble from MobileRead's Patricia Clark Memorial Library.
https://www.mobileread.com/forums/showthread.php?t=15837