Robert D. Richardson, the prize-winning historian and author known for his biographies of Henry David Thoreau and Ralph Waldo Emerson, died last week after a fall, the Associated Press reports. He was 86.

Richardson’s books focused on 19th-century American thinkers. His 1986 book Henry Thoreau: A Life of the Mind focused on the life of the Walden author and was called “splendidly written” by the Boston Globe.

Ten years later, he published Emerson: The Mind on Fire, which was a finalist for the National Book Critics Circle Award for Biography. He received additional critical acclaim for his 2007 book Willliam James: In the Maelstrom of American Modernism, which won the prestigious Bancroft Prize.

His most recent book, Nearer the Heart’s Desire, a biography of Omar Khayyam and Edward FitzGerald, was published in 2016. A reviewer for Kirkus called the book “an artful analysis.”

Richardson is survived by his wife, Annie Dillard, the author of Pilgrim at Tinker Creek and The Living. The two met after Dillard sent Richardson a fan letter praising his biography of Thoreau.

On Twitter, the Ralph Waldo Emerson Society paid tribute to Richardson, writing, “In 2001, the Emerson Society presented him with its Distinguished Achievement Award. A kind and generous scholar, Bob will be fondly remembered and greatly missed.”

And author Arlo Haskell, a friend of Richardson’s, tweeted, “What I learned about writing, research, history by interviewing him has carried me for years. His sparkling eye and goodness of heart warmed mine whenever I saw him over the fence or waiting for morning coffee.”

Michael Schaub is a Texas-based journalist and regular contributor to NPR.