AT&T Inc. committed malpractice in its stewardship of Time Warner, according to the man who led the sale of the parent of HBO, Warner Bros. and CNN to the telecommunications giant.

In a coming James Andrew Miller book about HBO, Jeff Bewkes, the then-chief executive of Time Warner who engineered the 2016 deal to sell the entertainment giant to AT&T for $85 billion, pulls no punches in his claims about how his successors managed the company. The book—“Tinderbox: HBO’s Ruthless Pursuit of New Frontiers”—is an oral history of...

AT&T Inc. committed malpractice in its stewardship of Time Warner, according to the man who led the sale of the parent of HBO, Warner Bros. and CNN to the telecommunications giant.

In a coming James Andrew Miller book about HBO, Jeff Bewkes, the then-chief executive of Time Warner who engineered the 2016 deal to sell the entertainment giant to AT&T for $85 billion, pulls no punches in his claims about how his successors managed the company. The book—“Tinderbox: HBO’s Ruthless Pursuit of New Frontiers”—is an oral history of the premium channel from its inception to present day, and is set to be released on Nov. 23. Mr. Miller conducted 757 interviews with more than 600 people for the book.

While Mr. Bewkes doesn’t express regret over the decision to sell to AT&T, he said he is angry about how his team of top executives and the staff were treated.

Among the high-profile departures were HBO’s former boss, Richard Plepler. Mr. Plepler left after clashes over strategy with John Stankey, who was tapped to run the entity—rebranded as WarnerMedia—and now is chief executive of AT&T.

“The most disappointing thing to me about the AT&T merger,” Mr. Bewkes is quoted in the book as saying, is that he and his board thought AT&T “would basically leave our people alone.” That didn’t happen, he said. “We didn’t think they would go to such a level of malpractice as to not listen to anybody…even though they themselves had no experience in those areas.”

When it took control of the company in 2018 after winning a prolonged antitrust battle with the Justice Department, AT&T significantly overhauled WarnerMedia. It consolidated much of its operations, including its stable of cable networks such as TNT and TBS. There was also a fair amount of instability among the executive ranks, as the team that Mr. Stankey had put in place to run HBO Max and Turner was soon replaced by his successor, Jason Kilar.

“If you are in an acquisition and somebody pays a premium for your stock, by definition it means something has to change,” Mr. Stankey told Mr. Miller in the book. “If you paid a premium for an operation and you continue to operate it exactly the same way, you never pay back the premium.”

John Stankey was tapped to run the entity, which was rebranded as WarnerMedia. He is now chief executive of AT&T.

Photo: Getty Images for WarnerMedia

Earlier this year, AT&T decided to unwind its big bet on entertainment by agreeing to combine WarnerMedia with Discovery Inc. into a new, publicly traded company.

Mr. Stankey said he still believes in the vision behind AT&T’s purchase, but said Wall Street and shareholders weren’t giving the company time to prove its strategy was sound, which led to the deal with Discovery.

“One of the jobs I need to do in carrying AT&T forward is ensuring we come up with a strategy that the investor base will tolerate and work through and give us the right credit for,” he said in the book, adding that the value of what AT&T created wasn’t being recognized. “They refused to give us credit for that progress,” he said.

In the book, Discovery CEO David Zaslav told Mr. Miller, “If we’re successful, and I believe we will be, there will be Harvard Business School case studies on this deal.”

Mr. Bewkes first planted the seed of a sale of Time Warner to AT&T in 2014, as he was looking to fend off Rupert Murdoch’s 21st Century Fox’s takeover attempt. (Part of 21st Century Fox is now known as Fox Corp., which shares common ownership with News Corp, the owner of Wall Street Journal parent Dow Jones & Co.)

According to Mr. Miller’s book, Mr. Bewkes bumped into

Randall Stephenson, AT&T’s then-chief executive, at the Allen & Co. conference in Sun Valley, Idaho, and told him, “our conversation ought to be with you” instead of Rupert Murdoch, Fox’s then-chief. Mr. Stephenson replied, “hold that thought…Maybe one day we’ll talk.”

Mr. Bewkes confirmed the accuracy of his quotes in the book, and so did representatives of AT&T (for Messrs. Stankey and Stephenson) and Discovery (for Mr. Zaslav).

Mr. Miller, who previously wrote tomes about ESPN, “Saturday Night Live” and Creative Artists Agency, interviewed HBO architects and cable pioneer Chuck Dolan, as well as stars and producers of practically every HBO program. Other key media figures featured include former Time Warner CEOs Jerry Levin and Dick Parsons, Comcast Corp. CEO Brian Roberts, cable executive John Malone and agent Ari Emanuel.

Write to Joe Flint at joe.flint@wsj.com