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.

“‘Sir,’ I said, ‘this Russia thing is far from over,’” Mr. Christie wrote. Mr. Trump responded: “What do you mean? Flynn met with the Russians. That was the problem. I fired Flynn. It’s over.” Mr. Kushner added, “That’s right, firing Flynn ends the whole Russia thing.”

Mr. Christie, who wrote that it all sounded “naïve,” recalled Mr. Kushner telling him that he was “crazy” when he said they would most likely still be discussing the Russia issue in February 2018.