Once he began debriefing Hussein, though, Mr. Nixon realized that much of what he thought he knew about him was wrong.

His most astonishing discovery was that by the time of the United States-led invasion of Iraq in March 2003, Hussein had turned over the day-to-day running of the Iraqi government to his aides and was spending most of his time writing a novel. Hussein described himself to Mr. Nixon as both president of Iraq and a writer, and complained to Mr. Nixon that the United States military had taken away his writing materials, preventing him from finishing his book. Hussein was certainly a brutal dictator, but the man described by Mr. Nixon was not on a mission to blow up the world, as George W. Bush’s administration had claimed to justify the invasion.

“Was Saddam worth removing from power?” Mr. Nixon asks. “I can speak only for myself when I say that the answer must be no. Saddam was busy writing novels in 2003. He was no longer running the government.” Strikingly, Mr. Nixon says that the C.I.A. had some evidence that this was the case before the invasion, but that “it was never relayed to policy makers and emerged only after the war.” By 2003, Mr. Nixon writes, Hussein’s disengagement meant that he “appeared to be as clueless about what was happening inside Iraq as his British and American enemies were.”

With Hussein increasingly detached, Mr. Nixon says that by 2003 Iraqi foreign policy decision-making had fallen to his lieutenants, led by the “unimaginative and combative” Iraqi vice president, Taha Yassin Ramadan, who “repeatedly missed opportunities to break Iraq’s international isolation.”

Regarding Iraq’s supposed weapons of mass destruction, the justification for the 2003 invasion, Hussein admits to Mr. Nixon that it was a mistake for him not to make clear before the war that he had long since gotten rid of them. “Saddam turned philosophical when asked how America got it so wrong about weapons of mass destruction,” Mr. Nixon writes. He quotes him as saying that “the spirit of listening and understanding was not there … I don’t exclude myself from this blame.”

Hussein never understood the United States, and Mr. Nixon describes him as repeatedly mystified by American intentions in the Middle East. After the Sept. 11 attacks, Hussein fatally misread how America would react. He thought the attacks would bring the United States and Iraq closer together to jointly combat Islamic extremists.