I think the operative is that there is not as high a percentage of pro-Bush books available. In fact, I am unable to name even one off the top of my head, whereas I can list a half dozen anti-Bush books. I don't recall any pro-Clinton books during his presidency, but I can think of a couple anti-Clinton books written by former aides. I think in general people tend to be more vocal (or writer-ly) when they are dissatisfied than satisfied.