Christopher Steele, the former British spy behind the partisan-funded and unproven “Steele dossier” ahead of the 2016 election, is reportedly writing a book about former President Donald Trump and Russia.

The book will be published by Mariner Books on Oct. 8 and is titled, “Unredacted: Russia, Trump, and the Fight for Democracy,” according to Politico, which first reported on the project.

Mr. Steele rose to prominence with a series of memos, or a “dossier,” that claimed Russia had a bevy of compromising information about Mr. Trump. It was prepared on behalf of Fusion GPS, a firm that received money via Hillary Clinton’s campaign through a high-powered Democratic law firm.

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The dossier was based on still-unsubstantiated raw intelligence reports. It was published by Buzzfeed News in January 2017 without Mr. Steele’s permission, leading to controversy and criticism from other media outlets.

Special counsel Robert Mueller ultimately took over a formal FBI probe and did not find evidence of a criminal conspiracy between Mr. Trump’s campaign and Russian operatives.

Mr. Steele’s new book continues to focus on Mr. Trump and Moscow, claiming Russian President Vladimir Putin “is now desperate to have Donald Trump back in the White House,” according to an excerpt in the “Playbook” newsletter.

“If Putin succeeds in helping Trump get reelected, I am convinced that the global political order will be utterly changed,” the book says.

Mr. Trump has blasted claims that he has ties to Russia, saying the allegations were hard on his family and saddled his presidency with formal investigations.

Earlier this year, a U.K. court dismissed Mr. Trump’s lawsuit against Mr. Steele.