Readers are right to flinch whenever a book under review is called, “magisterial.”

Yet there is a certain majesty in the way author Joseph Lelyveld combines his long-honed reporting experience with a historian’s eye firmly fixed on this important story. In this case it is an exploration of the labyrinthine mind of Franklin D. Roosevelt as he enters the decline leading to his death on April 12, 1945 at his hideaway in Warm Springs, Ga.

Mr. Lelyveld’s story is important on several counts. With an impressive array of new archival evidence he challenges the long-lived slander that FDR gave away Eastern Europe to the tyrant Josef Stalin either because (as many old Cold Warriors swear) Roosevelt was a Communist dupe or, more plausibly, because he was unaware of his deteriorating health and could not focus on the critical Big Three negotiations that he, Winston Churchill and Stalin waged at Tehran in December 1943 and again in Yalta in February 1945.