Terrible book. Sloppy scholarship - replete with misattributed quotes, misreadings, caricaturisations, historical claims made in it are factually incorrect, and he even misidentifies several modernist philosophers that themselves are critical of postmodernism as postmodernists. This isn't a work of serious scholarship, and how far it lies outside his narrow realm of specialisation in epistemology shows in how poorly its analyses and claims hang together.
It's as thinly veiled and horrifically puerile screed against those he counts as his political ideological opponents, a masturbatory attempt to congratulate his uncritical treatment of his own opinions, as much as Terry Goodkind's Sword of Truth series were and almost as equally a work of fantastical fiction; as a fellow Ayn Rand acolyte, he's in good company with Goodkind.
I can foresee only three reasons to write a book like this:
1) To serve as a text for his political allies to point to for their justification of dismissing postmodernism without ever engaging with a real, substantive representation of it while leaning on the authority of his credentials, however unrelated to history or political philosophy (one need only look to the Discovery Institute for such similar practices).
2) It's a cynical attempt to get in on the ground floor of monetising academic controversies with apocalyptic language to foment fear in those susceptible to such emotional volatility.
3) Or maybe, ironic for him being an epistemologist, he is so certain in his views because he is ignorant of his own ignorance and the human propensity to rationalise their own prejudices away. Perhaps he sees these as only the tendencies of other minds.