“Mr. Caro is continuing his work on Volume 5 with limited interruption,” Caro spokesperson Paul Bogaards said Thursday, a day after Gottlieb’s death at 92.

No release date has been set for what’s supposed to be the final book in “The Years of Lyndon Johnson” series, the first of which was published in 1982. Late last year, Caro told The Associated Press he had no sense when the fifth volume would be done, saying then that he still hoped to visit Vietnam and spend time in a village subjected to U.S. attacks during the Vietnam War.

Caro had worked with Gottlieb — also the editor of such celebrated authors as Toni Morrison, Joseph Heller and Nora Ephron — for more than a half-century. Their collaboration began with the Pulitzer Prize-winning “The Power Broker,” continuing with the first four Johnson biographies, and a brief volume on Caro’s writing process, “Working.”

Their unique collaboration was the basis for an acclaimed documentary, “Turn Every Page,” directed by Gottlieb’s daughter, Lizzie Gottlieb, and released last year.

As Caro told the AP ahead of the documentary’s release, he and Gottlieb did not have the kind of ongoing communication many writers and editors have. Caro did not consult with Gottlieb while writing his books or show him early drafts, waiting instead to turn in a completed manuscript. They saw so little of each other in social gatherings that Lizzie Gottlieb would say that she only got to know Caro through making the film, which does depict the pair editing “Working.”

The two Bobs argued fiercely over “The Power Broker” — when hundreds of thousands of words were cut from what still ended up as a 1,200-page book — but had since enjoyed a much warmer and more seamless working relationship.

“From the day 52 years ago that we first looked at my pages together, Bob understood what I was trying to do and made it possible for me to take the time, and do the work, I needed to do,” Caro said in a statement Wednesday — adding that he looked upon Gottlieb as a “great friend.”

The Johnson books, which have brought Caro a Pulitzer and a National Book Award among other honors, have become an obsession with history fans. Publisher Alfred A. Knopf receives more queries from fans about the fifth volume, expected to cover the last nine years of Johnson’s life, than about any other book.

Caro is a famously thorough researcher and expansive narrator whose books usually exceed 600 pages and can take up to a decade or more to finish. His most recent Johnson book, “The Passage of Power,” was released in 2012.

“You can’t speed up the research,” Caro told the AP last year, saying the scope of the final volume was “huge.”

Bogaards declined comment on who might edit the final book, which the 87-year-old Caro had acknowledged was shadowed by his and Gottlieb’s ages. Caro has said he sometimes receives mail from fans, urging him to finish Volume 5 and reminding him, “Do you know how old you are?”

“Bob and I don’t sit around and talk about it,” Caro told the AP last year. “We know how the clock ticks.”