Three main themes emerge in the book in relation to Mr. Trump.

Mr. Trump is less of a cartoon figure than he is in most accounts contained in new books about the White House. But Mr. Christie describes him as averse to interpersonal conflict with people he likes, needlessly nasty to some subordinates and prone to trusting people he should not.

Mr. Kushner, whose power has grown recently, appears as a shadow campaign manager and chief of staff in the White House, often giving his father-in-law questionable and problematic advice, according to the book, on topics including Mr. Flynn; how Democrats would perceive the firing of James B. Comey as F.B.I. director; his initial support for the campaign chairman, Paul Manafort; and how West Wing and key cabinet jobs were filled.

And a number of unqualified figures attached themselves to Mr. Trump and pandered to Mr. Kushner, Mr. Christie said, particularly after he was dismissed from the transition team. One was Mr. Manafort, who bluntly told Mr. Christie in the spring of 2016 that he was succeeding over a rival campaign aide “because I’m smart enough to agree with Jared, and he is not.”

On Feb. 14, 2017, Mr. Christie and his wife, Mary Pat, had lunch scheduled with the president. It happened to be the day after Mr. Flynn — whom Mr. Christie did not back for the national security adviser role — was dismissed for lying to the vice president about his contacts with the Russian ambassador during the transition. Mr. Kushner decided to attend.

As Mr. Kushner tucked into his “typical salad,” Mr. Christie wrote, the president said to him, “This Russia thing is all over now, because I fired Flynn.” Mr. Christie said that he started laughing, and the president asked why.