Amazon calculates a product’s star ratings using a machine learned model instead of a raw data average. The machine learned model takes into account factors including: the age of a review, helpfulness votes by customers and whether the reviews are from verified purchases.
I wasn't sure if I'd like this as it's not my usual genre, and it took me a while to get into it. But a few chapters in I'd gotten to know the characters and was really enjoying it- both the writing and the story. I ended up racing through it and can't wait for the next one (it seems to end with a potential for more so I do hope there's a sequel).