His new novel “My Mrs. Brown” just hit stores, but William Norwich has already optioned the screen rights, with a self-imposed Memorial Day deadline for the script.

Never mind that he has a full-time role at Phaidon Press and has started penning a sequel as well as another book. Twenty years after he published “Learning to Drive,” Norwich said he is amazed to have written a novel in 2016. “Just getting a simple sentence together from a very dizzy mind was always kind of a challenge. They don’t come out simple — sometimes they don’t even parse. I call it ‘The Wild Kingdom Between the Ears’,” he said.

Perhaps more striking is the fact that Mrs. Brown is a Rhode Islander of a certain age who embodies modesty, restraint and manners. While those traits don’t exactly spell out fashion, her only quest is to buy an Oscar de la Renta dress similar to one that belonged to the grande dame of her two traffic-light town. In a Q&A about the Simon & Schuster book with Kate Betts on Tuesday night at the Upper East Side Barnes & Noble, Norwich, whom many in the crowd know better as “Billy,” said the idea for the book stemmed from unearthing a copy of Paul Gallico’s 1958 novel “Mrs. ‘Arris Goes to Paris” in a thrift store in 2010. (Reader response to his New York Times tribute about Letitia Baldrige was further incentive.)

Set in post-World War II London, the little-known novel highlights how a charming cleaning woman becomes transformed by seeing a Christian Dior dress in a closet and is determined to buy one. That rainy Saturday afternoon read made Norwich wonder what type of designer dress might inspire a similar quest today. Once his own book was about to be shopped around to publishers, he discussed it with Alex Bolen, chief executive officer of Oscar de la Renta, who later received a first draft too. Norwich said his immersion in Mrs. Brown advanced from wondering what she might think of situations in his own life to responding to questions with, “Well, Mrs. Brown thinks…” So much so that Norwich said his partner, the artist Garrett Chingery, suggested she might be his alter ego.

As for Brown’s spirituality, Norwich told Betts, “Mrs. Brown’s character is the character of a lot of nice people. They just don’t talk about it so I talked about it on her behalf. My experience has been it isn’t as easy as it looks. I don’t think it’s as easy as it looks for anybody. There have been a lot of challenges. I feel at some point, I was either going to become the world’s biggest b—h or I was going to upgrade and tap into all the wonderful people who have been around me helping me and take that message. It was almost a choice. I’m getting some nice feedback about what a nice book it is, and I’m almost thinking, ‘Wait, am I nice?’ But I have a philosophy, an applied philosophy.”

His well-received second-grade poem about how a bird actually flew into his school cemented his plans to be a writer. Moving to New York to be a poet years later, Norwich wound up teaching school for a few years. When he was about to be rewarded with a fifth-grade homeroom, he was asked what he saw down the hallway of life. “I said, ‘I see Mick Jagger.’ Then I was unemployed for years,” Norwich told Betts. “When my friend Susan Minot got into the Columbia Fiction and Poetry program, I thought, ‘Well, I have to do that too.’” There, Mona Simpson and Edmund White helped him glean his writing skills and he later delved into fashion, helping columnist Eugenia Sheppard before moving on to The Daily News thanks to his mentor Liz Smith.

Peggy Siegal, who will host a Casa Lever lunch for Norwich with Samantha Boardman and Derek Blasberg on Monday, is his executive producer of choice for the movie version, although she hasn’t been told that yet. Ditto for Jessica Lange, whose strong suit for showing an inner life in her acting would ring true for Mrs. Brown, said Norwich, who lined up Barry Weiner and Matthew Saver to option the film rights.

Norwich will also be the guest of honor Thursday night at the Oscar de la Renta flagship on Madison Avenue, where Eliza and Alex Bolen are hosting a party. Another is planned for the Los Angeles store May 10. He will also be signing books at Bookmarc on April 28 with Christy Turlington, with some proceeds supporting her Every Mother Counts charity.

What Norwich is shooting for readers to learn is “that being an American grown-up is actually an honorable thing, that balance is OK, that loneliness is inevitable and that underneath the superficial there is a life for all people. And it’s not going to be what you see or what they say. Quiet people are the ones you want to love and know. Quietude is good after all the noise we’ve gone through culturally,” he said.