Briefing Donald Trump as he prepared to enter the White House was more difficult than briefing any other incoming president, a book published by the CIA said.

The publication, "Getting to Know the President," by John L. Helgerson, a former intelligence officer, charted different presidents' interactions with intelligence agencies.

In it, Helgerson wrote: "For the Intelligence Community, the Trump transition was far and away the most difficult in its historical experience with briefing new presidents."

Helgerson's book was published by the CIA for US officials. The CIA says in the book that its contents are the author's, and do not reflect the position of the CIA or any other US government agency.

Helgerson wrote that the only comparable experience to Trump's transition was that of President Richard Nixon.

"The only (and imperfect) analogue was the Nixon transition," he said, after Nixon "effectively declined" to work with the intelligence community and instead got his information through an intermediary.

“Trump was like Nixon, suspicious and insecure about the intelligence process, but unlike Nixon in the way he reacted," Helgerson wrote.

"Rather than shut the IC out, Trump engaged with it, but attacked it publicly," he added.

Helgerson's book was first published in 1996. It has been updated with each new president.

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