If you leave a poor review for a book, do the author a favor and tell them why. In some cases it might simply be subjective, since there are few stories everyone loves once they get past Winnie the Pooh. If it’s technical, such as too many typos, say so, giving the author a chance to correct it. Same goes for other things that were confusing or threw you out of a story.
Authors tend to take their reviews seriously and often use them as a guide for future revisions or as lessons learned for future books. However, no one should expect everyone to love their book. You can’t please all the people all the time.