Phyllis Hattis and William Rubin preferred living separately for the first twenty-four years of their twenty-six-year courtship. Hattis was an art adviser. Rubin, who died in 2006, was the director of the Museum of Modern Art’s department of painting and sculpture—maybe the world’s most powerful curator. She liked her independence. He liked his rental. He lived at the top of a forty-eight-story building on the Upper East Side, where he kept his personal art collection. “He was proud to tell me that he had the highest salary of any curator,” Hattis said recently. “But it was still low enough to need rent control!” Eventually, in 1990, the other portion of the penthouse came up for sale. Hattis relented, and pulled some cash together to buy it. “I sold a Picasso,” she said.

Their conjoined apartment—specifically the art they continued collecting in it—is the subject of a new book by Hattis, “Masterpieces: The William Rubin Collection—Dialogue of the Tribal and the Modern.” The book presents the works via a tour of the apartment: Picasso above the piano, Matisse by the tool drawer, tribal masks on the windowsill. (There are also personal notes from Frank Stella and Richard Serra.) The other day, Hattis offered a real-life trip through the penthouse.

“So, we had a little disaster yesterday,” Hattis said, emerging from the kitchen. She was wearing green corduroy pants, a gray sweater, and a gray scarf, and was carrying a carved mask. “We had a sculpture fall on the floor,” she went on. Her Pomeranian-Siberian husky, Banksy, was lobbying for a belly scratch. “We opened the door to the balcony so he could go pee. A gust of wind blew it forward.”

She walked into a living room. The square footage was ample. The carpet was a little worn. Buzzy lights, modernist furniture. “Le Corbusier, Le Corbusier, Mies van der Rohe, Mies van der Rohe,” Hattis said, pointing to couches and tables.

She stopped in front of a Picasso over a daybed, flanked by a headdress from the Baga people of Guinea and a mask from the Songye, in Congo. “These are the big guns,” she said. Rubin bought the headdress from a collector, with a briefcase full of cash. “A masterpiece,” Hattis said. The Picasso depicted the artist Françoise Gilot, Picasso’s lover, bent over a drawing. Hattis once asked Gilot about two medallions on her apron, in the place of breasts. “She said, jokingly, ‘He used to tell me that my tits were drooping after we had two children,’ ” Hattis said.

Rubin became friends with Picasso when he acquired his Cubist sculpture of a guitar for MoMA, in 1971. After Picasso died, Jacqueline Roque, the artist’s wife, came to the opening of one of Rubin’s MoMA shows. “She got to the door, and the guard said, ‘I’m sorry, the museum is closed,’ ” Hattis recalled. “She said, ‘But I’m Jacqueline Picasso.’ And he said, ‘And I’m Jesus Christ.’ ”

Rubin started his collection with some money from his father, who owned textile mills. He eventually got a loft in lower Manhattan and filled it with Abstract Expressionists. Hattis pulled out some photographs of the space. “This is Rothko,” she said. “This is Motherwell. That’s Frankenthaler. This is Larry Poons. He sold the Pollock to build a house in the South of France.”

After Rubin moved uptown, and Hattis bought the neighboring unit, he proposed combining apartments. “I said, ‘We can have an adjoining door,’ ” Hattis said. At one point, they almost split up. “So we closed the door,” she said. “That lasted a couple of weeks or something. Then we opened the door.”

She continued into another wing, opening a heavy door. The old bachelor pad. Banksy trotted with her. They turned a corner, coming to a wall-size Stella color-field mural, near a Stella relief painting. Hattis isn’t particularly interested in selling. (“To be an investor collector, where the art is stored in warehouses and in free ports to avoid taxes, I’m not that,” she said. “That’s a shame.”) But the bigger pieces, like the relief painting, required some sacrifices. “I could’ve put a television screen there for movies!” she said.

Onward: an Arp, a Matisse, a Warhol inspired by a Matisse. Near a desk by the balcony were two kafigeledjo figures from the Senufo people of West Africa. “These two guys are my buddies,” she said. She calls one Max, for Max Ernst, and one Jean, for Jean Dubuffet.

There were some actual Dubuffets, too. One was in a box under the big Picasso. It used to be on the wall, but she hadn’t found the right spot to rehang it. “I miss it,” she said. There was open wall space right next to little Jean, if she shifted over a Matisse. “Let’s just do it,” she said. She produced a hammer and began whacking. “What do you think?” She stepped back to appraise, and furrowed her brow. “Let’s live with it for a while,” she said. ♦