I found a recently published technical book on Amazon, which had a couple dozen five star reviews. I read a few of them, and it seemed great. The author's biography said he worked at X, a very well-known software company. I bought a paperback copy, read it, and was disappointed. The book wasn't terrible, but it wasn't written very well, and did not contain the technical depth that it appeared it would on the reviews. The book turned out to be self-published, and the technical editor was the author's boss at X. The other editors were the author's family members.
I went back and looked at the reviews, and found that many of the five-star reviewers worked at X, or if I couldn't figure out their employer, they happened to be located in the same metro area as X's headquarters. One of the Amazon reviewers is even mentioned in the acknowledgements of the book.
I think all the reviewers had good intentions to help their friend and colleague, but I think it's still misleading, as you cannot expect a someone to give an impartial public review on their colleague's work.
All in all, the book was not total junk, but perhaps should have been a 3/5 star book instead of 5/5.