McClellan's 400-page book, "What Happened," describes how in October 2003, he faced reporters every day and related what he knew about who leaked the name of a CIA agent to a syndicated columnist and its political ramifications, CNN reported.
"There was one problem. It was not true," McClellan wrote. "I had unknowingly passed along false information."
McClellan, who resigned in April 2006, went on to describe how he spoke based on information given him by U.S. President George Bush, Vice President Dick Cheney, presidential adviser Karl Rove, vice presidential aide Lewis Libby and Chief of Staff Andrew Card.
In March, Libby was convicted of perjury and obstruction of justice for lying to investigators and a federal grand jury about his contacts with reporters. Rove left the White House at the end of August and was not charged.
McClellan's book is to be published in April.