This is my opinion and its what I and some readers do. When we come across a book that is by an author we know and like, sometimes we automatically buy the book and read it. Then we write and look at other readers' reviews and agree or disagree.
If it's a book by an author we don't know, there are no reviews and the preview for the book seems ok, we either add the book to our wish list and wait till someone has an opinion or some of us will take a chance and buy the book. But let's be real, sometimes readers don't like doing that because they may feel like they wasted their money on a book they don't like by someone they don't know and tends to may not want to try another book by the author but this is only sometimes.
If it's a book by an author we don't know and it has reviews, we tend to investigate the reviews to see what the reader is about. I know when I'm on Amazon or Goodreads and I'm thinking of buying a book from an author I don't know, I immediately go to the reviews. Of course I look at the negative and positive reviews.
***For the positives, I would click on the reviewer's (see all my reviews list). If I see that this person has a set amount of reviews which for me could be more than 10 or 20 reviews within the past couple of weeks then I will surely consider what they said. Seeing this I know I can trust their opinions on the book they are reviewing. This could also work for me for the negative reviews sometimes. But to make it be known when a person only has 3 reviews within the past months or year, no good the review seems fake.
***For the negative reviews, I try to not let that sway me a lot. I do do the same process as I would the positive reviews. When it comes to the negatives and I'm seeing constant negatives, I'm not able to trust that person. Why keep reading the same books over and over that is not to your liking just so you can leave constant negativity.
I would say this to authors, which I'm no expert, just saying. When you do pick a reviewer to read your work, make sure that reader has a certain amount of reviews to be taken seriously. Readers don't just look at the 5 or 4 star reviews of that reviewer, they look at more. Also use someone that just doesn't use 2 or 3 sentences saying and writing I loved it and thought it was great! Sometimes you want to know why it was great. You would be surprise the difference reviews make in purchasing a book.