James Bradley’s incendiary new book about Theodore Roosevelt is not really packed with secrets. Much of the material it discusses has long been hidden in plain sight. But Roosevelt biographers often subscribe to certain orthodoxies, and one of them is this: When Roosevelt made noxiously racist and ethnocentric remarks about Anglo-Saxon greatness, so what? He was just voicing the tenets of his time. Mr. Bradley, the author of “Flags of Our Fathers,” does not simply cite Roosevelt’s egregious talk. He presents this much-ignored aspect of Roosevelt’s thinking with sharp specificity (“I am so angry with that infernal little Cuban republic...