With the unfortunate exception of Donald Regan, time was that presidential staffers waited at least until their bosses -- the men who had "brought them to the dance" and offered them unparalleled opportunities to serve, and to witness history first-hand -- were out of office before dumping on them (or their wives, as in Regan's case). Such delicacy no doubt entailed some sacrifice in terms of remuneration; in order for memoirs to sell well once a president's out of office, they have to offer insights of actual value, rather than simply serving as a hatchet-job account designed to make a quick buck -- and that's a much more daunting proposition.
As Matt points out below, Scott McClellan has damaged his long-term economic prospects and his reputation by apparently adopting positions that are antithetical to those he held (or at least voiced) when he was working at The White House. But even aside from the questions about his judgment and his loyalty that this kind of about-faced memoir raises, Scott McClellan has created a David Brock-style credibility problem for himself (named in honor of the man who has penned both The Real Anita Hill and Blinded by the Right, within eight short years of each other).
The fact is that when you offer two versions of the truth, you make it clear that at least one time, you've been lying.