Perhaps it was meant to be. For out of that failed assignment emerged five years later Let Us Now Praise Famous Men, a nearly 500-page meditation on poverty, injustice, and the mysteries of the universe. The book sold only a handful of copies in Agee’s lifetime, and was long out of print when its author — a chain smoker, a hard drinker, and a suffering soul — died in a New York City taxi in 1955 at the age of 45. Only after Agee’s posthumously published novel, A Death in the Family, won the Pulitzer Prize did Famous Men find an audience, and ultimately its place in the American literary canon.

Famous Men is a beautiful book, but it is not an easy read. What finally got me through it was a Fortune assignment of my own: to return to Hale County on the occasion of Fortune’s 75th anniversary in 2005 and reclaim a part of our history. (“Best break I ever had on Fortune,” Agee wrote of his assignment; I would say exactly the same of mine.) The result was “The Most Famous Story We Never Told,” and while I was reporting it, I searched in vain for Agee’s original manuscript. The best clue I had about what was in it was Evans’s remark years later that “Half unconsciously, and half consciously, Agee saw to it that it would not get into Fortune.” I assumed that whatever he had come up with was unpublishable as magazine journalism.

I was wrong. Thanks to some enterprising scholarly detective work, we can now read what is almost certainly the long-lost Agee manuscript, published this month in book form by Melville House as Cotton Tenants: Three Families. “The cotton belt is sixteen hundred miles wide and three hundred miles deep,” Agee begins, in clear, straightforward prose. “Sixty percent of those whose lives depend directly on the cotton raised there, between eight and a half million men, women and children, own no land and no home but are cotton tenants.”

What extends from that opening is a masterpiece of the magazine reporter’s art. It is lucid, evocative, empathetic, deeply reported, consistently surprising, plainly argued, and illuminated, page after page, with poetic leaps of transcendent clarity. Agee’s description of a sharecropper’s house, for instance: “entirely of pine, stitched with nails into as rude a garment against the hostile year as a human family can wear”; and of night descending on that house: “A man may wake, coughing in darkness; a child may cry, and be quieted; on the porch a dog may burst up bellowing from a nightmare and set ten miles of country echoing with his kind …”

In his excellent introduction to this volume, Adam Haslett describes Agee as one among the tribe of “social visionaries and broken-hearted artists” whose “work, in the manner of Jesus strained through Marx, insists that distinctions between the suffering of intimates and the suffering of strangers are an outrage.” That, in the end, may have been what did Agee in, at least so far as his career as a writer for a glossy business magazine was concerned.

This story is from the June 10, 2013 issue of Fortune.