When it comes to folks discovering their next read, word of mouth is best. This could be as simple as a friend shoving a book under your nose and yelling READ THIS BOOK! or you might encounter a review in your local paper. Or, you may belong to an online book club on Facebook. Or you trawl Amazon or Goodreads for books in genres you enjoy reading. Ten to one, before you make your choice, you're going to see what other people have said.
Reviews are, therefore, gold for authors (especially if they're placed on vendors' sites), and a wonderful thank you if you've enjoyed a literary work. (Apart from buying a book or writing a bit of fanmail.)
I've had folks come to me saying they don't know the first thing about what to write in a review. Likewise, I've had authors commenting that readers end up giving a blow-by-blow report of the novel's events rather than share their opinions in reviews. So, here I'm going to give a few tips on how to write a decent review and talk about some of the pitfalls of offering your opinion.
YOU DON'T HAVE TO RE-INVENT THE WHEEL
There is no need to rehash the plot. By all means, say a little bit about the characters and the circumstances, to give a (brief) description but be careful to avoid spoilers. As an example, this is my way of introducing a review of Goldilocks and the Three Bears: Goldilocks is a nosy little girl who doesn't respect others' private property, but on the day that she visits the Bears' household, she's in for a little more than she bargained for.
That's enough. It's fine. It sets the scene. Now, ask yourself the kinds of questions you imagine other potential readers might ask.
What did you like about the story?
Mention here that the plot was full of surprises. Perhaps you were up until late reading (authors love hearing this). Perhaps the writer uses poetic language or is really good with descriptions. Perhaps the dialogue between characters is especially witty and made you laugh. Mention things like this.
Were the characters well-realised?
Characters are either likeable or unlikeable. Don't just say that you hate something or love something. Tell readers why. You have your own personal likes and dislikes, so be aware when these may colour your opinion. Characters may do things you don't agree with or like, but that doesn't mean you're reading a bad book.
Was there anything about the story that annoyed you?
This could be anything from the fact that a novel is poorly edited and there were lots of errors or that the writing itself was simplistic. Perhaps characters' behaviour lacked sufficient motivation. Don't just say something was awful; try to figure out what bugged you. Merely saying "This is a terrible book" won't explain to potential readers why you think it's terrible.
When you write your review, remember that there is another person on the receiving end of your words. It's all too easy nowadays to let rip. While I'm not advocating that you only say nice things, my suggestion is that you sandwich the positive with the negative so that your review is balanced. You might not like that the novel is written in the present tense, for instance, but you enjoyed the way the author depicted the setting.
Take notes while you read so that you have a record of your responses. (Goodreads offers functions for this.) I always love comparing my review to others just before I post it to see if there are other reviewers who've felt similarly. You may find yourself agreeing or disagreeing, and that is fine – each reader will have a unique response to a written work.
One thing I always keep at the back of my mind when writing is that I may very well one day meet the author whose work I've read, so I aim to keep my tone respectful. If I can't share my opinion to their face, then there's a chance that perhaps I should not write it down online (where it may well be carved in stone).