In early February 1861, President-elect Abraham Lincoln sat down in his Springfield law office to write his inaugural address. Since his election the previous November, seven states had seceded from the union, and he knew that nobody would care about his administration’s plans for internal improvements or tariff reform; all the nation wanted to know was what he would do about the rebellion.

The logical first step was to explain why secession was illegal, but as Penn State professor Mark E. Neely Jr. writes in his engrossing new book: “He obtained a copy of the Constitution from his law partner. When …