Any clue about William Shakespeare’s life usually excites scholars, but one piece of evidence had been neglected for decades. Now, a new analysis of that overlooked document seems to shatter a longstanding narrative about the Bard’s bad marriage.

Shakespeare was 18 in 1582 when he married Anne Hathaway, the daughter of a family friend in Stratford-upon-Avon who was in her mid-20s and pregnant. For centuries, it was thought that the writer left his wife and children behind for a literary life in London, seeking to avoid “the humiliation of domestic feuds,” as one influential 19th-century essayist put it.

This view of Shakespeare’s wife as a “distant encumbrance” suited scholars who thought “Shakespeare was far too interesting to be a married guy,” Matthew Steggle, a literature professor at the University of Bristol in England, said in an interview. The perception was bolstered by the fact that Shakespeare had left her his “second best bed” in his will.

But Mr. Steggle’s new research, published on Thursday in the journal Shakespeare, suggests that the writer was not detached from his marriage after all.

The hint lies in a fragment of a 17th-century letter addressing a “Mrs Shakspaire,” found in the binding of a book published in 1608. The letter’s existence was noted in 1978 by an amateur historian, but it got minimal attention, even after the book was unbound in 2016, revealing what appeared to be part of a reply from Shakespeare’s wife, Mr. Steggle said.

He was working on a Shakespeare biography when he learned of the 1978 find, and was surprised it wasn’t better known. Technological advances allowed him to track down people mentioned in the long-ago correspondence, along with other evidence indicating that it included the playwright’s wife, he said.

If the letter really was addressed to the Mrs. Shakespeare, rather than a lesser-known person with a similar name, “it is self-evidently remarkable,” Mr. Steggle said. It not only gives some previously unknown Shakespeare contacts, but also offers new clues about their relationship, and even suggests that Mrs. Shakespeare lived for a time in London with her husband.

If Hathaway did live in London, she was possibly back in Stratford by the time she received the letter, likely around 1607 — though not necessarily because her husband wanted independence, according to Mr. Steggle.

He proposes in his paper that “there is an obvious reason to avoid London in 1603-4, namely the very bad wave of plague.” In addition, the upcoming arrival of the Shakespeares’ first grandchild after their daughter Susanna’s 1607 marriage “would surely be a good time” for Hathaway to be based back in Stratford.

Mr. Steggle suggests that Mrs. Shakespeare’s movements should be reconsidered with an eye to her “possible absences from London rather than her perpetual absence.”

The note to Mrs. Shakespeare concerned money for a fatherless child named John, who was an apprentice, though not under the famous playwright, with the last name “Butte” or “Butts.” It called upon her to pay money that was most likely held in trust for him, a pledge that her husband may have undertaken, and it referred to a time when she “dwelt in trinitie lane,” which Mr. Steggle now believes refers to a location in London.

The book that held the letter was a 1608 text printed by Richard Field, a native of Stratford who was Shakespeare’s associate, neighbor and first printer, according to Mr. Steggle. Wastepaper was commonly used in bookbinding, and “given Field’s extensive known links to the Shakespeares,” the discovery of their family documents in a work he published indicates it was likely addressed to the famous Mrs. Shakespeare, Mr. Steggle said. Notably, the response, which appears to come from her, sounds “organized, businesslike and rather sarcastic,” he added.

As for John Butts, the child in the letter, his name did appear in a 1607 record of an institution that disciplined disobedient apprentices, among other records, and Mr. Steggle said his surname did arise in “Shakespeare’s extended personal network.”

“The stakes are high,” Mr. Steggle writes in his paper. “This letter, if it belongs to them, offers a glimpse of the Shakespeares together in London, both involved in social networks and business matters, and, on the occasion of this request, presenting a united front against importunate requests to help poor orphans.”

His findings lend some heft to feminist readings of Shakespeare’s life and a pop culture trend, as seen in the popular stage musical “& Juliet,” as well as the acclaimed novel “Hamnet,” of rethinking the marriage and Hathaway’s role in it.

In the musical, Hathaway comes to London from Stratford to attend a performance of “Romeo & Juliet” and annoys her husband by suggesting an alternate ending in which Juliet does not die. The novel depicts the couple’s complex relationship and their shared grief over the death of their son.

Like those reinterpretations, the four-century-old letter undermines long-held premises about the playwright’s life. For Shakespeare biographers “who favor the narrative of the ‘disastrous marriage’” — and even those who do not — the document “should be a horrible, difficult problem,” Mr. Steggle’s paper concludes.