Everyone thinks they can write a good book review. "Good" in this case is not subjective. Either a review is well-written or it isn't.
In order to write a good review you need to be familiar with the anatomy of a good review.
Book reviews must tell the reader what the book's theme is about. Simply saying," interesting book" isn't good enough. Your job as a reviewer is to tell the reader what the book is about, how you feel about the book, etc.
This is why when writing a review, the first part of your review must contain the theme of the book. Imagine that you've read a book and want to tell your friend about it, what would you tell that friend?
Obviously you'd tell them what the book's story is about and why you liked the book. And then you'd either recommend the book or tell them not to waste time reading that book.
That is a simple way to write a relatively objective review.