Christopher Hitchens will go on trial later this month in a "highly critical" new book which interrogates the late polemicist's politics and argues that this celebrated left-wing firebrand became an "amanuensis" of the George W Bush administration in his last years.

Political activist and author Richard Seymour's Unhitched: The Trial of Christopher Hitchens is out on 28 January and promises to cast "a cold eye over the career of the 'Hitch' to uncover an intellectual trajectory determined by expediency and a fetish for power". "It is written in the spirit of a trial," said Seymour. "I do attempt to get a sense of the complexity and gifts of the man, but it is very clearly a prosecution, and you can guess my conclusion."

Unhitched will address how Hitchens moved from a "career-minded socialist" to, post 9/11, a "neoconservative 'Marxist'", said its radical publisher Verso, and "an advocate of America's invasion of Iraq filled with passionate intensity". At one point, Seymour describes Hitchens as the "George W Bush administration's amanuensis", and argues "that not only was Hitchens a man of the right in his last years, but his predilections for a certain kind of right-wing radicalism – the most compelling recent example of which was the Bush administration's invasion of Iraq – pre-dated his apostasy".

"One chapter deals with the trajectory of his political shift, from the time he was a young socialist who joined Labour," said Seymour. "I've interviewed a lot of his former comrades. If you read [Hitchens' memoir] Hitch 22, it's not an entirely reliable account – what he remembers and what others remember are different. He's subtly, and sometimes not so subtly, revised things."

Seymour will also examine Hitchens' views on "war and empire, and his surprising shifts on that front – he's not as consistent in that shift as he liked to make out in retrospect", said the author. "And there's a chapter on his approach to religion, and his approach to the 'English question' – Thatcher, and Orwell."

While there are aspects of Hitchens that Seymour admires – "there are parts in his writing where you read it and glow, it's so perfectly put," he said – the book is, overall, "a denunciation of the changes he underwent in the last 10 years in particular, with Iraq and America the two central themes".

The book was commissioned about six months before Hitchens' death in December 2011, although Seymour had previously written an essay about the author, which was included in the collection Christopher Hitchens and His Critics. Seymour had been exchanging emails with the author, and sent him a copy of the essay. "We stopped exchanging emails shortly afterwards," he said. "He thought of it as an insult, and threw a few back."

But Seymour is hopeful that if he was alive today, Hitchens "might have had a bit of a laugh" about Unhitched. "One thing in his favour is that he was narcissistic but not prickly or vain," he said. "I think he would have thrown an insult or two at me. He described Max Blumenthal as 'a young skunk who hasn't learned to piss yet' and I think I could expect something along those lines."