If you see a book with a lot of reviews, does it make you want to buy it? If a 'review' simply says 'it's great', or 'buy this book' is that enough for you to want to spend money to read the book?
A good book review will give the reader something to think about before they decide whether or not it is for them. Reviews are important to authors as certain algorithms show that the more reviews a book has, the more sales the book generates. Great books inspire readers so much that they want to talk about them and what better place to spread the word than to write a review and post it in a prominent place. Similarly, if you feel the author could have improved the book in some way, and you have some useful comments to contribute, leave a constructive review. If you feel disposed to write a review for a book you have just read, and aren't sure how to do it, here is a simple guide.
Read the book. This seems obvious, but I have seen 'reviews' where people have said 'I didn't finish the book' and then give it a very low rating. I have started some novels that are just so badly written that I can't waste another minute of my time on them, but I just move on to the next book in my reading pile. If you don't read the book, don't leave a review.
Don't feel obligated to give a glowing review. You can always tell the reviews that have been written by friends of the author. They usually have five stars and talk more about the author's talent than give the reader any insight into the content of the book. Were you unsatisfied with the ending or did the blurb not accurately describe the story? For you to leave a review, the book must have had some impact on you, so state the facts simply and leave out the gushing adjectives.
Be constructive with your criticism. Whether you enjoy reading the book or not, there will be some people, places or events that stand out. If you feel there is too much dialogue, or not enough description, then say so in your review, but also point out anything you may have learned from reading the book or any characters that you could identify with in some way. Did the book make you laugh or cry? Was it a gripping read that you couldn't put down? Did you like the author's writing style? Whatever you say will have an impact on other readers and the author, so give your view but be kind.
Don't leave a one sentence statement such as 'it's great' or 'buy this book'. It doesn't help anyone.
As a reader, I have written reviews for a lot of great, good, and those-that-have-potential books. As an author, I have received a lot of great feedback in the form of reviews from people who have enjoyed my books. You get people who take issue with a particular subject because it offends their particular tastes, and that's OK as long as they say that in their reviews. When I am looking to purchase new books, I look at the five-stars and the two-stars to get a good overview of readers' thoughts.
Keep writing book reviews, but be kind, and if you can't be kind just move on to the next book. A well-written book review will always be appreciated by an author.