Most of Koonin’s sources are directly from the IPCC reports, in fact they are what the book is based around.
And, yes, I do not agree with every point he makes. I think some of the things he says are misleading. That said, he does raise points that climate action advocates, myself included, need to address. For instance, our faulty climate models. This is not climate denial. It’s climate action, from a realistic perspective.