Clarke is certainly no liberal. He's a hawkish conservative who has worked for the Reagan, Bush I, Clinton, and Bush II administrations. Since the book is very critical of the current administration, they have worked hard to claim that his criticisms are "sour grapes" for being passed over for a job. Read the book and judge for yourself. My reading is that this is an enormously competent man who loves his country and who sees its defense being handled badly. He feels that it is his duty to speak up, and I agree.
In my opinion, the final chapter is the most important. Here is the summary paragraph from the beginning of Chapter 11 (p.247):
"What would a successful and comprehensive counterterrorism effort have looked like after September 11?
It would have consisted of three key agenda items. First, the President would have engaged in a massive effort to eliminate our vulnerabilities to terrorism at home and strengthen homeland security. Second, he would have launched a concerted effort globally to counter the ideology of al Qaeda and the larger radical Islamic terrorist movement with a partnership to promote the real Islam, to win support for common American and Islamic values, and to shape an alternative to the popular fundamentalist approach. Third, he would have been active with key countries not just to round up terrorists, end the sanctuaries, dry up the money, but also to strengthen open governments and make it possible politically, economically, and socially for them to go after the roots of al Qaeda-like terrorism. [The priority countries are Afghanistan, Iran, Saudi Arabia, and Pakistan.] Nowhere on the list of things that should have been done after September 11 is invading Iraq. The things that we had to do would have required enormous attention and resources. They were not available because they were devoted to Iraq."
He develops these points in more detail in the rest of Chapter 11, and draws on the background presented in the preceding parts of the book.