British art historian John Richardson, known for his multi-volume biography of Pablo Picasso, died today at age 95. A representative from Knopf, Richardson’s publisher, confirmed the news.

The author’s long-anticipated fourth volume of A Life of Picasso is complete and scheduled to be published later this year, according to the Art Newspaper. “A publication date for the final volume has not been set,” the Knopf spokesperson told artnet News. “We will assess it all after the dust settles.”

Richardson was born in London in 1924. He studied at the Slade School of Fine Art and briefly went on to do window design for Harrods, as well as some textile designing. When it became clear that making art wasn’t in the cards for his career, Richardson turned to art criticism, writing for the New Statesman, the left-leaning British newspaper.

He then met art historian Douglas Cooper, who would become Richardson’s partner for ten years. They moved to Provence in the south of France, where Cooper introduced Richardson to Picasso in 1948. A close friendship developed between the two men.

Picasso “was very generous to me. He sensed somehow that I was going to write about him. He had a very strange understanding of his work,” Richardson told journalist Alain Elkann in 2016. “When I decided to write about him, originally it was going to be about a specific part of his life, his portraiture, about the people he met, going back to the early years.”

John Richardson’s home with a portrait of him by Andy Warhol. Photo by François Halard, courtesy of Rizzoli.

Richardson was also a close friend of Andy Warhol. He appeared in one of the Pop artist’s films and gave the eulogy at his memorial. Queen Elizabeth II knighted Richardson in 2012. (Her great-grandfather, King Edward VII, previously bestowed the same honor on his father, Wodehouse Richardson.)

In the 1960s, Richardson opened the New York office of Christie’s auction house and ran it for nine years before joining New York’s Knoedler & Co. gallery. The biography on Picasso became Richardson’s primary focus in 1980.

The first volume, The Prodigy, 1881–1906, was published in 1991 and was named the Whitbread Book of the Year. The Cubist Rebel, 1907–1916 followed five years later and the third volume, The Triumphant Years, 1917–1932, appeared in 2007.

“As I get older and older, I get more and more astonished the deeper I get into his work,” Richardson told ARTnews in 2012. (At the time, he was aiming for a 2014 publication date.) Some reports have suggested that the final volume will cover the rest of Picasso’s life, but in 2016, Richardson told Elkann that he was considering concluding either in 1940 or at the end of World War II, in 1944. (Picasso died in 1973.)

This month Rizzoli is publishing John Richardson: At Home, a book showcasing his various residences around the world, their “Bohemian Aristocrat” interiors, and his personal possessions, which include photographs, correspondence, and art from Picasso himself.

“He would sometimes embellish the letters his wife Jacqueline wrote to me or Douglas [Cooper] with little vignettes,” writes Richardson in the book.

Richardson was also in the process of curating an exhibition of Warhol’s celebrated society portraits for Gagosian Gallery in London, which is expected to open this year.

“In every conversation with John, he taught you something new—he could reveal things about a painting and its history that no one else could know. It was magical,” said Larry Gagosian in an email to artnet News. “The depth of his knowledge was astounding. It’s not just the passing of a friend, but the passing of an era. We won’t see another like him.”