Look, the same market forces that have the White House press corps, which always tries to get access, even potentially more craven than usual are the market forces that give outside reporters a strong incentive to get the stories that the White House press corps can't and won't get. Those kind of stories don't depend on access, anyway; they use sources, but sources who are motivated to leak things that the administration doesn't want aired, not sources who are motivated to make the president look good. They also depend on the kinds of digging that are as easy or, most of the time, easier to do away from the White House than from inside it. And they aren't subject to a bias far more pervasive than the book incentive, as this excellent Lane Wallace article explains. Overall, I'd say that the "problem" of White House reporters writing books is a prime example of something not to worry about.