Originally I was going to move on to the positive things in a book that would help me decide to choose it, but I feel a little more about reviews themselves should be brought up. While I'm sure that I am not the best at writing reviews I know what I need in order for a review to have meaning. So here is my list of 9 review do and do nots.
1) Never start a review with "I never do reviews, but just had to for this book". That really doesn't add more weight to your words. The fact you have never bothered to write a review more than likely simply means you shouldn't have written this one either. This is different than one just being your first review instead. We all have first, but there is a big difference between your first review and saying you had to for THIS book. Even if I did believe you it wouldn't make a single difference in my decision.
2) One of the worst things to say in a review is "I don't want to reveal the plot so I'm not going to talk about it". This simply means you are not going to tell the reader anything that will help them to make a decision. That is sort of the point of a review to give the person interested in the book enough information (without too much) so they can make an informed decision of whether they might like to purchase it or not.
3) Interject some actual meaning into what you say. I mentioned this before, but if you take twenty paragraphs and actually say nothing then it isn't a review. Comments like "if you like fantasy books you will like this one" or "the imagery was amazing" mean next to nothing. The first is pure opinion and gives the reader no basis on which to help make a decision. It is great you think because someone likes on fantasy book they will like another, but that generally isn't the case. The problem with the second it has no substance. What kind of imagery and how was it amazing are more important than the fact you thought it was amazing.
4) You are writing a review not auditioning to write Blurbs for the back of books. A good review is going to consist of more than a hundred words. Usually this will fill up quickly if you follow the previous step and actually interject some actual meaning into what you say. Still it is quite possible to add meaning and still only come out with a Blurb. The difference is in the amount so here I mean that you should say something and before the idea is to make what you said meaningful. Also, paraphrasing the synopsis isn't a review it is simply lazy.
5) Using your word-a-day dictionary will not only prevent you from sounding as sophisticated as some people seem to want to appear, but also points back right the previous in that you may not actually be interjecting any meaning it to what you are saying. It is great that you might know words like "hyperbole", "symmetry", "overwrought", "analogy", "simile", etc. The problem is these are generic descriptors and have no depth of meaning if you don't add actual context. How is the author's use of hyperbole meaningful? If they simply use things like "The bodies piled around him were so high as to almost touch the ceiling" or use of simile in "His punch made me feel like I got hit with a ton of bricks" then while in context they have meaning this doesn't mean the author is the master of hyperbole and simile. If you are going to make such a comment as the author uses hyperbole then again make your comments have meaning. This could be something as simple as an actual line from the book showing your example.
6) In a multiple review environment you should have more to your review then the last person. This is about quality not quantity. If you say the exact same thing or paraphrase the last person then you are wasting your time and others. It would be more meaningful simply to say look at this persons review instead. That isn't to say you can't agree with them and mention some of the same points, but you should have more to your review than that. You can mention that your review is in addition to another one if you feel what they said summed it up well. This would then leave you room to spend time on giving more information rather than rehashing what has been said.
I found 45 reviews of a book I was interesting in reading. One manage to tell me two things more than the synopsis and the other 44 managed to say the same thing as that one review. The sad fact is that still not a single one of them managed to tell me anything really significant about the book. One example of what would be nice to know is that very few books are actually only about one person so if there are significant supporting characters they should be mentioned as well as their relationship to the main character.
7) If a plot point makes you uncomfortable then mention it even if you have to be vague. If something really put you off or if something made you give 4 stars out of 5 then it could be something someone else might not buy a book over. You might also say if you are aware of something that might offend others even if it doesn't offend you then it would be nice to mention it.
8) A review is often about the quality of the whole not just part of it. The meaning to this is with ebook versions sometimes done haphazardly it could have actually text problems. I've seen tilted pages words from one sentence appearing halfway down the page, and whole pages missing. Some feel that to give a low review for this isn't acceptable which sadly isn't the case unless you can rate them separately. A review is everything about a work and you can mention that the quality of the overall finished product isn't up to the quality of the writing, but by not mentioning and rating accordingly you are doing a disservice to everyone. If substandard is acceptable what is the point in doing it better?
9) Some people like spoilers and some do not so note when you put spoilers in a review. Personally I have no problem with spoilers and there have been many books I would not have purchased if I could have found spoilers about them. However we should understand that others might feel differently so while you can add spoilers to your review (please do) note them so others are not put off by it.
Remember while you may have your various reasons for writing a review the reason most people read them is so they can have a better understanding of something they are interested in buying. Whether it is a book, movie, or a power tool the end results are the same so being thoughtful and informative are the most important things. Should these 9 rules be the absolutes of review writing? No, that isn't what I'm trying to say. Everyone is going to have their variations on the formula to a perfect review and some of their points might be even more valid than what I listed here. Still these hit those things that will make a review worth while to those that are looking for help making a decision on what to read rather than more wasted space one has to bypass while looking for that help.