Though Mr. Cunningham lived to see greater acceptance, he became modest and effacing in his self-presentation, usually wearing a French sanitation worker’s blue jacket and khakis to capture the more flamboyant citizens of New York. He submitted reluctantly to a 2010 documentary.

“It feels like he had internalized that reaction,” Mr. Richards said of the disapprovals of his childhood. “It’s speculation to think of why he decided not to publish this in his lifetime, but my assumption, having spent a lot of time with the text, is because though he really wanted to tell the story of this special period in his life, his education in creativity and style, at the same time he was worried how people were going to respond.”

But aside from some scenes of family discord, Mr. Cunningham’s memoir is a rosy account of an irrepressible dreamer who tripped his way from the stockroom of Boston’s newly opened Bonwit Teller to hat shops of his own in New York. He arrives in the city in November 1948 on opening night of the opera — then a tent pole of the New York social calendar — and stays long after the Social Register stopped being anyone’s bible.

Much of the material is new, even to his relatives. “Bill kept his family life in Boston and his work life in New York very separate,” wrote his niece Trish Simonson, in an email. “He told us stories over the years, but nothing that painted a full picture of what he did and how he came to do it. The drafts of the memoir we found, titled and edited and written in his own unmistakable voice, filled in a lot of blanks of how he made it from here to there, and what he thought along the way.”