Time has a way of sanding off the rough edges of historical memory, turning even the most convulsive, contentious lives into opportunities for national triumphalism and self-congratulation. With “Frederick Douglass: Prophet of Freedom,” the historian David W. Blight wants to enrich our understanding of an American in full who, for more than half his life, wasn’t even legally recognized as such. Now that Douglass is enshrined on his pedestal, shorn of what made him “thoroughly and beautifully human,” Blight notes how the “old fugitive slave” has been “adopted by all elements in the political spectrum,” eager to claim him as their own.

Plenty has been written about Douglass in the 200 years since he was born, not least by Douglass himself, who recounted his life story in three autobiographies — a paper trail of memoirs that Blight deems “both a pleasure and peril” for the biographer. In tracing an arc from bondage to freedom, Douglass cast himself as a “self-made hero,” Blight writes, while leaving “a great deal unsaid.” A number of other books have filled in the gaps — exploring Douglass’s relationships with the women in his life, for instance, as well as his fraught and transformative friendship with Abraham Lincoln — but Blight’s is the first major biography of Douglass in nearly three decades, making ample use of materials in the private collection of a retired doctor named Walter O. Evans to illuminate Douglass’s later years, after the Civil War.

Blight, who has edited and annotated volumes of Douglass’s autobiographies, undertakes this project with the requisite authority and gravity. The result is comprehensive, scholarly, sober; Blight is careful to tell us what cannot be known, including the persistent mystery of Douglass’s father (who was most likely white, and may have been Frederick’s mother’s owner). On the stuff that’s known, Blight is an attentive if sometimes fastidious guide, poring over speeches and texts with the critical equivalent of a magnifying glass. Douglass, Blight says, was a “man of words,” making this book “the biography of a voice.”

That voice took shape and sharpened over time, but it would return again and again to the banks of the Tuckahoe River on Maryland’s Eastern Shore, where Frederick Augustus Washington Bailey was born in 1818. Twenty years a slave, then almost nine years a fugitive; as Douglass himself described it in his autobiographies (having adopted his new surname from a Sir Walter Scott poem), the first decades of his life were both thrilling and terrifying. Until his abolitionist allies helped to purchase his freedom in 1846, everything he did felt provisional; he lived with the incessant fear of someone who could be plunged back into captivity at any moment.