Before buying a book, I often check out its reviews on Amazon. But I strike a problem. It is the bias of self-selection. People who really like a book will put in the effort to review and rate it. People unmoved by the book will (I'm guessing) simply try to forget it. A few, truly offended/enraged people will damn it in a review. My point is, because reviews are optional, I think they're positively slanted.
So how many positive reviews do I have to see before I buy a book? Not sure. But the rare 2 star review, if the reviewer gives a reason, will often stop me buying a book. Atrocious copy editing, plastic fashionable language, a spoiler that reveals something awful are all good reasons to save my money.
Which begs the question (horrible phrase) why am I including reviews on this site?
Because reviews are useful, and just as useful is the hints to other books. If a reviewer has liked three books you like, what are the chances you'll also enjoy the fourth book they review? Probably fairly good. So I hope my reviews bring to light books that are a joy to read.