It's a needlessly insulting little book, and at least partially incorrect. Still, it carries some interesting ideas; too bad they weren't better written and researched.
Here are a couple of points out of many where this book is pretty awful:
1) His description of evolutionary biology and the problem of fossil ancestors is just uninformed. He makes the common error in assuming that we humans have evolved and improved, but our ancestral species have not. Then, he wonders "where are the ancestral species?"
Silly! The ancestral species have evolved too. It's basically symmetric: we and the apes both evolved from a common ancestor. We are as much the ancestral species of a chimpanzee as chimpanzees are our ancestors. The reason that ancestral species are not here any more is simply that all species change with time because their environments change and because of genetic drift. It's only the very few species that live in quiet, stable environments that travel though history and change but little.
Now, this is not the central point, but it's well known and has been well popularized by people like Stephen Gould, so it's something that Harper should have known when writing the book. You read this and then wonder "what other errors are there?"
2) He tells a story about how academics might avoid dealing with inconsistencies in their data. It sounds plausible enough, and may even sometimes be partially true. But, academics are not just fat cats who are trapped in their boyhood myths. Most of us are curious and want to know what is really going on.
Most of us understand that conforming to the standard model is indeed a good way to live quietly and comfortably, but we also know that there is nothing better than breaking the standard model, if it can really be proven to be wrong. The model breakers are the people who are remembered by history and the ones who get the juicy academic posts and prizes. The quiet people who conform may live comfortably, but they tend to live comfortably in second rate, out-of-the way institutions.
So, while there are forces for conformity in academia, there are also forces for revolution. If an academic discipline slides into slothful conformity, you can be sure it is because real proof is unobtainable, not because people are too blind to see it. If there were clear evidence, some ambitious junior lecturer would grab it, and use it.
So, don't take the book too seriously. It's probably wrong. There's certainly no known way to prove it right. Still, it has an interesting idea or two in there. Do we really know that the common people in AD 800 spoke Anglo-Saxon? Do we really know that in 55 BCE they spoke British (i.e. a Celtic language)? How do we know that they didn't speak something rather closer to modern English?
I don't know the answers, but I'll keep an eye open. Just in case he's right, it might give my career a boost.