Authors pour their heart, time, and effort into writing their book. They then spend hours searching and speaking to many publishers to find the best possible way to distribute their work without giving all of the profit to the publisher. Sometimes they publish independently!
The Author finally manages to publish his work, but no one knows how good the book is unless someone decides to press that star button or write something nice about it. This is the harsh reality of how things work.
Here is where a nicely written review comes in handy. The first thing you probably should know is that everyone is interested to see what you think. Hence, giving an honest and detailed review makes it clear for the buyers and provides constructive criticism to the writer.
When you decide to take on reviewing a book, the first thing you do is taking notes. Every chapter you read will leave an impression on you. It may give you certain feelings or have you question certain things. These are all excellent points you keep for when you decide to write the review.
If you wait until you finish the entire book and then start writing, you may miss certain parts of the book or not be able to give a fair judgment on the entirety of the work. However, if you give it a day and then read your notes, you are more likely to make a better judgment and better understand how good or bad the book really is.
Time to Review
First, do not give away the ending!
There are two kinds of reviews you can write:
A Descriptive Review
Aims for the purpose of the work and provides a description mentioning some passages from the text. Make sure the description is intriguing so that the reader will want to know more. Also, try talking about the character, theme, plot, style, setting, biography, and history of the work.
A Critical Review
Aims for knowing more about the author and their purpose in writing such a book. The book should represent what the author does or is about. This relativity between the author and their book is normally the case for all. However, there are times where they do not sync. In any case, if they do, then you have more to say. You have to be comfortable knowing the entire context, literary standard, and history of the genre of which the book stands before you can even consider basing any evaluation on the work.
The work content should talk about the things you liked and the things you thought could have been better. If you cannot give constructive criticism towards the book, you better not say anything.
Do not forget the Author has spent hours of time and money and his whole heart into the story. Your deconstructive criticism will only give false negative feedback on the book, and you potentially become a factor for a possible cause of a good book becoming underrated. I am sure many may disagree with me here, but I firmly believe that unless you do not have a really negative fact about the book, then you best not say anything at all.
When you wish to summarize, emphasize the good parts of the book rather than the bad. You have already stated the negative part, and there is best not to dwell on it too much. If you wish, you can consider rating the book based on how you liked or disliked the book; you can also suggest that the reader buy the book or not consider it at all.