There is nothing authors like more than reviews, but many readers are reluctant to review because they are not sure what is expected. No, you don’t have to give the entire synopsis of the book. In fact, that’s a great way to piss off potential readers who don’t want the spoilers.
Instead, mention the title and genre of the book, how much you enjoyed it, and anything that makes it particularly attractive to a potential reader. And yes, it’s fine to say what you didn’t like.