I’ll take issue with both statements.
“Verified purchase” does not equate to reliability, especially in regards to books on controversial issues, as people pushing a particular viewpoint are likely to give raves to books that support their views, no matter how badly written or poorly documented. By contrast, non-verified purchasers who’ve obtained books through other sources (including the public library) often write thoughtful, detailed reviews that are worth reading (full disclosure: I have posted a number of reviews on Amazon of books about vaccination, pro and con, which I put a good deal of effort into even though I didn’t buy the books on Amazon).
Ignoring all the five-star and one-star reviews is counterproductive (exceptions being one-line reviews consisting of unexplained adoration or loathing). I’ve also noticed people gaming the review system with three-star reviews which Amazon considers negative. The trick used is to say something glowing about the book but only give it three stars. If enough people “like” the review, it shows up as the leading “negative” review - obviously a deliberate attempt to prevent people from seeing a truly negative featured review.
Online reviews are always going to be somewhat of a crapshoot, seeing that they can be manipulated by zealous fans and non-fans. I have had better luck with Amazon book reviews than with (for example) many restaurant reviews. No matter how ordinary or even crummy a restaurant is, there seem to be plenty of people on Yelp, TripAdvisor etc. who think it’s the cat’s ass.