The best way to write a review is as if you are telling a dear friend about the book. You are not “reviewing” the book and passing a judgement about it being good or bad or trying to give it a rating score (4/5, 8/10, etc.). It should be a much more intimate experience. Like you are discussing the book with a buddy, talking about what you liked, what you did not like, how you felt while reading it, would you want your friend to read it, what do you think about the author, her style, etc.
Conversations are not about the number of words in it, do you start a discussion thinking – I will cover this topic is 500 words? Similarly, reviews should not be about its word count. Tell what you want to tell – no matter how many or few words it take.
The structure you should follow depends on the book and on you – for some books, you may want to start with the author, for some with the narrative, you may want to talk about the characters first for some, or may be an absolutely brilliant dialogue can become your starting point. What ever you think can start an interesting conversation.
Also, the most important thing to remember is that you are not selling the book, you are helping someone make a choice whether to commit his time and money to read it. As with a friend, be very frank, highlight both the good and the bad points and leave the decision to the reader. This frankness is something you owe to your readers – and you have no obligations (to give a positive review) to author, publishers, PR guys – even if they have provided you a review copy.
Writing reviews should make you as happy as discussing books with friends. Enjoy your next review, the way you enjoy chatting about a book with you buddies over a cup of coffee.