He was writing fluff for the campaign not a serious book. He was writing "John Kerry as told by John Kerry.
If Kerry wins Brinkley will be set for life. He will be Kerry's official biographer his career as a historian and an academic secure.
If he reveals what he knows and tells the truth before the election, Kerry loses and no one will give a rats rear end about Kerry biographies and he will be a pariah in the left wing world of academia, he will not get any TV show invites.
His problem is if he withholds the facts and continues to cover for Kerry, the truth will come out some day and he will be disgraced and he will be remembered as someone who betrayed his profession.