Please note: I did not read the book, hence I am posting a comment and not a review. This being said, I understand that Suskind doesn't offer any proof other than claims that a couple of CIA officials told him that they were ordered by the White House to forge and backdate a letter. The two CIA officials have denied telling Suskind this.
Despite this sort of shaky "sourcing," all of the reviews here thus far have accepted the claim unquestioningly. One wonders how Suskind and his "reviewers" here would react if they were accused of committing a serious crime like Suskind is accusing the White House of committing and the sourcing for that accusation was the same sort (no documentary evidence and statements that the witnesses deny making). Something tells me that Suskind and his admirers here would be screaming about how unfair it was and would be running to their lawyers to sue their accusers for libel.
As always, I think it is amazing to watch people slam the White House for cherry picking Iraq intelligence and then turn around and do the exact same thing (accept the most shaky of facts) simply because it fits their world view.