"In the permanent campaign era, it was all about manipulating sources of public opinion to the president's advantage," McClellan says.
McClellan was also critical of the White House's early "state of denial" in the face of the devastation from Hurricane Katrina and the government's response to it.
"Top White House officials who knew the truth, including Rove, Libby, and possibly vice president Cheney, allowed me, even encouraged me, to repeat a lie"
McClellan began working for Bush when he was the governor of Texas.
He served as chief White House deputy press secretary from January 2001 to July 2003, when he became the president's lead spokesman.
He resigned, or was pushed out, in April 2006, his credibility damaged by a scandal over the leaking of information that Iraq war critic Joseph Wilson's wife, Valerie Plame Wilson, was a CIA agent.
McClellan accuses former senior White House political strategist Karl Rove and Lewis "Scooter" Libby, once a senior aide to Dick Cheney, the US vice-president, of misleading him into publicly denying they played any role in the revelation.
Bush was also deceived, "but the top White House officials who knew the truth - including Rove, Libby, and possibly vice-president Cheney - allowed me, even encouraged me, to repeat a lie", says McClellan.
However Brad Blakeman, who worked alongside McClellan as a deputy assistant to Bush between 2001 and 2004, told Al Jazeera the former press secretary was making the allegations for money and had violated the trust of the US president and the office for which he had worked.
"Why so far after he left office is he complaining about things in his book when he was serving the president," he said.
"If he saw these things going on and he believes [it] so strongly he should have left at that time and told the American public his grievances at that time, not now.
"What Scott is saying is that the president used his position to sell a war that was unjustified and I reject that."
'Puzzled and sad'
Dana Perino, Bush's current press secretary, issued a statement rejecting the book's claims.
"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."
McClellan concludes the Iraq war "was not necessary" and admits that some of his own words on behalf of the White House 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.
The book is due to be published on June 1.