Historian Robert Dallek on his book "Franklin D. Roosevelt: A Political Life"

I've read 4 FDR biographies these last few years. This was a very good one because it uses contemporary source material to piece together FDR's mind along the path of his life. Dallek is a highly respected biographer.

Other biographies: FDR by Jean Edward Smith, who is excellent at synthesizing material and seeing them in a new way.

Nigel Hamilton's FDR at war Trilogy. Hamilton, incredibly, stayed in the Churchill household as a child, and found that Churchill wrote his own biography after the war, but FDR of course died. So he uses source material to follow FDR's decision making process as Commander in Chief. FDR ran the war, often against the advice of his military advisors and War Department.

"No Ordinary Time" by Doris Kearns Goodwin deals mostly with the Depression era in the FDR administration.

The world would be a different place today if FDR had not outwitted Hitler, or, according to Hamilton, listened to Churchill.