The intellectually honest thing is to assume not that the author is maliciously incompetent, but that you, the reader are missing something and inquire with the author about sources or reasoning for claims that contradict the apparent scientific consensus.

If the author doesn't respond to such an inquiry within ample but reasonable time, then you can go loading up the shotgun with accusatory birdshot and take it to the court of public opinion.

That's the last resort.

Some people (lots of people) really are just bullshit artists, and it's a waste of time to consult with them personally when their thoughts are already out there, in the form of a published book.

BS. Too many people write too many books. See the Brandolini's law.