"The Accidental President," by A.J. Baime, subtitled "Harry S. Truman and the Four Months that Changed the World."
When running for his fourth term, FDR selected Truman, a long-serving senator from Missouri, as his running-mate. FDR was inaugurated on January 20, 1945, and died suddenly less than three months later. When Truman succeeded to the presidency, the war in Europe was grinding to its close, but the war in the Pacific was still raging, and the Cold War was in its infancy stages. And FDR left Truman woefully uninformed and unprepared for the presidency -- including failing to tell him about a certain top secret project underway.
It's fascinating to read about how Truman, thrown into the maelstrom, was able to rise to meet the many challenges.