PHOENIX  President Bush “convinces himself to believe what suits his needs at the moment,” and has engaged in “self-deception” to justify his political ends, Scott McClellan, the former White House press secretary, writes in a critical new memoir about his years in the West Wing.

In addition, Mr. McClellan writes, the decision to invade Iraq was a “serious strategic blunder,” and yet, in his view, it was not the biggest mistake the Bush White House made. That, he says, was “a decision to turn away from candor and honesty when those qualities were most needed.”

Mr. McClellan’s book, “What Happened: Inside the Bush White House and Washington’s Culture of Deception,” is the first negative account by a member of the tight circle of Texans around Mr. Bush. Mr. McClellan, 40, went to work for Mr. Bush when he was governor of Texas and was the White House press secretary from July 2003 to April 2006.

The revelations in the book, to be published by PublicAffairs next Tuesday, were first reported Tuesday on Politico.com by Mike Allen. Mr. Allen wrote that he bought the book at a Washington store. The New York Times also obtained an advance copy.