I enjoy reading books other than my own and, knowing how import they are to both the author and prospective readers, I will often leave reviews for those works. How I construct those reviews depends on the content.
All art is subjective, and at the end of the day its relative worth will vary based on who’s offering their opinion. Regardless of whether I consider a story mildly interesting or absolutely enthralling, if I believe that it is well written and that others will find enjoyment in its reading, I will leave the highest possible rating (typically five stars.) If I don’t like the book then I will not leave any review at all.
Books which are not about make-believe places and people will be reviewed honestly, with scores and comments based on their factual accuracy. To do otherwise would be a disservice to both author and potential reader.