Hahaha I just looked it up and the CIA has an official position on the book. Don't know if this makes it more or less credible but I'm still down to read it. Check it Y'all:
Whoa. Apparently the CIA considers it credible enough to publish a rebuttal. I actually find it hard to believe they would do that. Why wouldn't they just ignore it?
It made a pretty big splash when it came out. The CIA had also just been "dismantled" by the Bush administration, itself at a low ebb, and the damn WMDs were very much not going to be found. Legacy of Ashes pretty much captured the lowest PR nadir of the CIA since Carter cleaned house, and it captured it at a time when the country was eager to read about large 3-letter agencies getting their comeuppance.
I've got no answer for you, but to be honest if the CIA didn't consistently do things that people found baffling they wouldn't be much of an intelligence agency.