Anything less than 4 stars means “NOT RECOMMENDED.” Don’t expect an author to be pleased with 2 or 3 stars, no matter how much you rave in the text. Those stars are the primary way a book is judged. Without a 4 or 5 star rating, a book doesn’t get picked up in the Amazon algorithms for things like “also bought” suggestions. Giving 1 or 2 stars to a book that doesn’t have many reviews is taking money out of the author’s pocket, so don’t do it unless you really think the author should take up a new line of work.