John and Abigail Adams. James and Dolley Madison. Jack and Jackie Kennedy. Bill and Hillary Clinton. Our history has been shaped by the talents and accomplishments of formidable first ladies, whose enduring partnerships with their husbands have shaped the thinking of presidents in myriad ways, creating legacies that mold us today.

No political marriage has cast a broader net of influence than that of Franklin D. Roosevelt and Eleanor Roosevelt.

In the third and final volume of Blanche Wiesen Cook's magisterial biography of ER, as she's called throughout the book, Cook turns her focus from Eleanor's inner life and journey toward independence, prominent in the first two volumes, and back to the Roosevelts' complex, ever-evolving relationship.

"Eleanor Roosevelt, Volume 3" opens in 1938, as the rise of Hitler and threat of war in Europe were pushing President Roosevelt and Congress toward dire choices. As a full-throated champion of the New Deal, ER now embraced a larger public role, continuing to write her newspaper column, "My Day," and stepping up her lectures and participation in Washington policy discussions.

Cook devotes an enormous chunk of her book to America's torturous debates about entering the conflict, FDR's own dithering and decision to run for an unprecedented third term — a gamble, given the strong currents of isolationism in the country. ER officially maintained a low profile when it came to FDR's agenda, but privately she wielded increasing clout, coaxing and chiding her husband to take firmer positions across a spectrum of domestic issues, from racial injustice to economic inequality to the scourge of anti-Semitism.

She occasionally tapped a finger on the scale of his foreign policy, as well. FDR welcomed her views, often relying on her counsel, but their frequent disagreements allowed him to withdraw from her and their five adult children, as well as a growing cast of grandchildren, which made ER feel diminished. Their bond was always affectionate and secure, despite his dalliances, such as his affair with Lucy Mercer Rutherfurd, which lasted until his death.

As Cook notes, "Whenever [ER] felt wounded by her husband's thoughtlessness, or became impatient or distrustful of his political strategies, she considered his heroic determination, his steadfast vision, and his unwavering pleasure in so many things. … She loved to watch him in the pool, where his strong arms and torso camouflaged his useless legs, and where he shouted and played with children."

"Eleanor Roosevelt, Volume 3" depicts ER's emergent public persona with more detail and anecdote than in the previous two volumes, and Cook's prose reflects that shift, with cool, crisp sentences that avoid her earlier worshipful tone. Even as Cook treads familiar history, her perspective, through ER's eyes, is vigorous and fresh, the comparisons with our own darkening world subtle and yet potent.

The book effectively ends with FDR's death, just weeks before V-E Day, with Cook packing ER's last 17 years into an afterword, a rich period in which the former first lady traveled extensively, advocating for human rights and the critical importance of the United Nations as a hub of international peace and goodwill. Despite this editorial misfire, "Eleanor Roosevelt, Volume 3" achieves the biographer's lofty goal: to bring ER to life through her own words and deeds, and charting, over the course of three volumes, her sumptuous arc: from reluctant socialite to devoted wife and mother to engaged political spouse to crusader for our better angels. It was an American life like no other.

Hamilton Cain is the author of "This Boy's Faith: Notes From a Southern Baptist Upbringing." He lives in Brooklyn.

Eleanor Roosevelt: The War Years and After, Volume 3
By: Blanche Wiesen Cook.
Publisher: Viking, 670 pages, $40.