White House aides seemed stunned by the scathing tone of the book, and Bush press secretary Dana Perino issued a statement that was highly critical of their former colleague.
"Scott, we now know, is disgruntled about his experience at the White House," she said. "For those of us who fully supported him, before, during and after he was press secretary, we are puzzled. It is sad - this is not the Scott we knew."
Perino said the reports on the book had been described to Mr. Bush, and that she did not expect him to comment. "He has more pressing matters than to spend time commenting on books by former staffers," she said.
"Scott himself repeatedly made the case for the war from the podium and even after he left the White House, I remember watching him on Bill Maher's show - about one year ago - making the case for the war," former White House Press Secretary Ari Fleischer wrote in a statement. "If Scott had such deep misgivings, he should not have accepted the press secretary position as a matter of principle."
In the book, entitled "What Happened: Inside the Bush White House and Washington's Culture of Deception," McClellan wrote that the Bush White House made "a decision to turn away from candor and honesty when those qualities were most needed" - a time when the nation was on the brink of war.
The way Mr. Bush managed the Iraq issue "almost guaranteed that the use of force would become the only feasible option," the book contends, according to accounts Wednesday in The New York Times and Washington Post.
"In the permanent campaign era, it was all about manipulating sources of public opinion to the president's advantage," McClellan writes.
The book provoked strong reactions from former staffers as well.
"For him to do this now strikes me as self-serving, disingenuous and unprofessional," Fran Townsend, former head of the White House-based counterterrorism office, told CNN.
Said former top aide Karl Rove, in an interview with Fox News Channel: "If he had these moral qualms, he should have spoken up about them. And frankly I don't remember him speaking up about these things. I don't remember a single word."
Richard Clarke, another former counterterrorism adviser who also came out with a book critical of administration policy, said he could understand McClellan's thinking, however. Clarke told CNN that he, too, was harshly criticized, saying that "I can show you the tire tracks."
McClellan called the Iraq war a "serious strategic blunder," a surprisingly harsh assessment from the man who was at that time the loyal public voice of the White House.
"As press secretary, I spent countless hours defending the administration from the podium in the White House briefing room," McClellan writes. "Although the things I said then were sincere, I have since come to realize that some of them were badly misguided."
"The Iraq war was not necessary," he concludes.
McClellan's book is "nothing but bad news" for Republican presidential nominee-in-waiting, according to CBSNews.com senior political editor Vaughn Ververs.
"The GOP candidate can (and probably will) dismiss McClellan's charges by pointing to the fact that, whatever came before, the war in Iraq continues and he's the one who can bring it to a successful conclusion," Ververs wrote. "But that's not going to make the war - or those who supported it initially - any more popular."
McClellan admits that some of his own words from the podium in the White House briefing room turned out to be "badly misguided." But he says he was sincere at the time.
"I fell far short of living up to the kind of public servant I wanted to be," McClellan writes. He also blames the media whose questions he fielded, calling them "complicit enablers" in the White House campaign to manipulate public opinion toward the need for war.
The book is scheduled to go on sale June 1. Quotes from the book were reported Tuesday night by the Web site Politico, which said it found McClellan's memoir on sale early at a bookstore.
McClellan draws a portrait of his former boss as smart, charming and politically skilled, but unwilling to admit mistakes and susceptible to his own spin. Mr. Bush "convinces himself to believe what suits his needs at the moment," McClellan writes.
He also faults Mr. Bush for a "lack of inquisitiveness."
"President Bush has always been an instinctive leader more than an intellectual leader," McClellan writes. "He is not one to delve deeply into all the possible policy options-including sitting around engaging in extended debate about them-before making a choice. Rather, he chooses based on his gut and his most deeply held convictions. Such was the case with Iraq."