This is a pretty good question. It obviously starts from knowing how to write. Because I feel before you can review a book, you must know what elements and aspects to look out for. And to know that you must of course be a decent writer yourself.
I'm not saying you must be a creative fiction writer or poet or Ernest Hemingway or anything. What I mean is simply that you must be able to know and identify the fundamentals of simple writings yourself, from there you can move on to attempting to review another person's book.
The first step to writing good reviews is familiarizing yourself with how good reviews are written. How do you do this? Read reviews of course. And I can say for myself that reading reviews is a hell of a lot more fun than most people think. I enjoy reading them immensely because for me it is one of the very best ways to improve your writing skills.
When you see how a book is reviewed; what was praised and what was criticized in the book, you learn to imitate and adapt your own writings towards it. But that is by the by. As a reviewer you're on the look out for something similar but towarfs another end--to learn how to write your own review. There is a lot you can learn from watching how the masters do their crafts.
The next step then, obviously, is to pick a book you have in mind to review and read it as carefully and as thoroughly as you can.
Now you should note that reading a book for your delectation is quite different from reading a book for review. When you read a book for review you are on the lookout for even the tiniest detail and you can't afford to miss a single detail like you can when youre reading for fun.
You have to note all the descriptions, the dialogues, the peculiarities; what makes it stand out and what is shared with others. Then when you're done--and only when you're done--would you know if you really should review the book or not.
Did it drive a passion in you? Did you feel a strong emotion all the way through it--either that of strong affection or loathing? Did it leave you totally unisipired? If you can't give a definite answer to any of these questions then you really shouldn't waste your time reviewing the book. But if you do, then write about it. Just write about it. Your experience.
Of course when doing the actual writing first thing you should do is summarize the book for your readers. And your summary must be done in such a way that people who haven'tread the book can be familiar with the story without giving much away, such as the ending or important plot twist.
You only need to summarise the basic settings and character makeup that sets the tone for the conflict and then you can move on to YOUR experience and thoughts on the books.
Tell the readers or even show them something in the book you saw that's hard for anyone else to see. Tell them your thoughts on the characters. Are they well developed? Are they shallow? Do they lack vitality? Or verrisimilude? Talk about the description; are they vivid enough? Vague? Is the WHOLE writing bloated? Brilliant?
Then you may compare the works with other works in the genre. Is it bad but still an improvement in others? Or is it entirely worse off? Or is it the best you've ever seen? Then you might even compare it with works from other genre. I remember the best review I ever read in my life was a review of Catcher in the Rye in Harold Blooms Modern Interpretations. I forgot who wrote it.
The best part about that review was how Catcher in the Rye was linked with the school to Taoist philosophy, and the whole eastern philosophy. While reading the book I did not notice this of course, but in the end after reading the review I finally saw it and it was so beautiful!
Now finally all these things should be done with tact, so that your review does not come off as just regurgitating random thoughts. All aspects must be systematically placed and incorporated into the review. And then introduce the unifying factor for all the aspects. Lay down your points and link them all in the end.
You need to find your own style in what you feel comfortable doing this.
I would like to separate a book review into sections:
• What's the book about? 1 paragraph introduction in your words
• Words that describe the book (adjactives, genre, etc)
• Your opinion of the book (Did it made you feel anything or analyze something? Did it made you think about life? Did it made you realize something? What did you like and what not?)
• Your favourite character/part in the book and why?
• Recommendation: Your rating in desired point system (5/5 or 5/10). Would you recommend this to someone? To which kind of people do you think the book would like? (ex. "If you are a romantic person, this book is definitely for you")
I would follow this simple format and then add or trim somewhere along the writing process:
1. Write a plot summary about what you are reviewing.
2. Make a commentary about the characters, settings, and plot execution using your own honest opinion.
3. Name the strengths of the book you are reviewing. Is it reader friendly? does it really answer the question what the reader wants to know? Is it really a book worth the rating? How well were the plot twists executed? How fast or slow was the pacing for you?
4. Along with number 3, state the weakness or areas you think the book could improve on. It's also not bad to state your version of the remedies for the flaws you identified.
5. Your recommendations. This part contains your overall rating. Do you recommend the book worth the read? to who do your recommend it to. You can justify why you gave such rating.