When I set out to write a new critical biography of Sylvia Plath nearly nine years ago, I knew I would need to devote a significant amount of time and space to another great poet: Ted Hughes. Plath, an American, and Hughes, a Brit, were married for nearly seven years beginning in 1956, and theirs was one of the most productive—and turbulent—literary marriages of the 20th century.

Many of Plath’s most famous poems, including “Daddy,” “Ariel,” “Lady Lazarus,” and “Purdah,” were written in the wake of Hughes’s departure, in the fall of 1962. Earlier that summer, Hughes had fallen in love with another woman, Assia Wevill. Hughes and Plath separated, and she had intended to divorce him, though the couple was still technically married when she committed suicide, in February 1963.

Hughes and Plath, 1956. Their rocky marriage would end with her suicide, less than a decade later.

Hughes stayed mostly silent about Plath’s death for more than 30 years, a position which led him to be reviled by some and lionized by others. The guilt and trauma he felt over Plath’s death, he told friends, nearly paralyzed his life and work. In 1998, he published Birthday Letters, a collection of elegiac poems about Plath that became a best-seller. He claimed that publishing the book had finally freed him. But it was too late. Hughes died that same year.

The marriage of Ted Hughes and Sylvia Plath was one of the most productive—and turbulent—of the 20th century.

During the course of my research, which spanned both sides of the Atlantic, I found that the poems of Birthday Letters were not all Hughes had written about Plath—far from it. Nothing quite prepared me for reading Hughes’s other unpublished poetic sequences about Plath, which are held in the Ted Hughes archive at the British Library, London. These poems are not secret—Hughes scholars have known about them for years—yet they remain hidden to all but the few researchers who have visited the archive in person.

Plath at work near the Beacon, Hughes’s family home, in Yorkshire, U.K.

Most astonishing to me was the “Trial” sequence that Hughes wrote in the 80s, when he was involved in a libel lawsuit in Boston over a film adaptation of Plath’s 1963 novel, The Bell Jar. In these poems, he remembers visiting Plath at her new London apartment to celebrate the publication of her novel; her worry over the book’s positive but dull reviews; her feelings about the book’s heroine, Esther Greenwood; his decision not to read The Bell Jar until after Plath’s death; and his promise to Plath’s mother never to publish the novel in America. He struggles to understand why Plath wrote The Bell Jar, and whether the act of writing and publishing it led to her suicide. As in Birthday Letters, he sees dark familial forces at Plath’s back, and sidesteps his own role in her tragedy.

The “Trial” sequence, scrawled with changes and excisions, offers a rare glimpse of Plath as Hughes remembered her last two years. It confirmed, for me, the value of “slow” biography—of long weeks spent in the archive, sifting through a life. The poems show Hughes still speaking to Plath, still trying to understand why she, in the end, left him behind.

Heather Clark’s Red Comet: The Short Life and Blazing Art of Sylvia Plath is out now from Knopf