Reviewing a book is easy. You open the book, you read the book, you finish the book and then you write your opinion on it. Your review may be on the book as a whole, on the story itself, the way it is written, or even in the cover only. At the end it does not matter, a review is an opinion and this opinion is personal and untransferable.
Now, there is people writing these reviews and these reviews exist because there is people reading them. People want to know what a book is about before reading it, they want to know if it will be worth the time, if they will get something out of it at the end. They are looking for something a preview will never provide as a preview will give you a glimpse of what is to come, but it will never tell you about how the book is written, or if the settings are credible, or if you will relate to the characters in any way.
Reviews provide a wider vision of what is inside a book and the tricky thing is that it comes straight from the reader’s point of view, subjectivity at its fullest you may say. But then, if this is the case, you may wonder: are reviews trustworthy?
Well, they are and they are not. They will never be objective of course, but they will give you a better and more reliable idea on the book in question, and just like with movies, if the amount of bad reviews is greater than the good ones, then why even bother. But of course, and being fairly honest, if you want to know if you would like a book, you should just go an read it yourself, because at the end of the day your opinion is the only one that will matter.
As I see it, reviews are tools for readers, and they should only be seen as such. Once you start doing some research and building a little community, you start to know who to trust and who shares your same –or similar- vision, and that is when you start to choose whether to read or not to read a book based on a review, and not before.
However, you should never judge a book by its cover (although marketing has improved a lot in the past five years), and you should never base your thoughts on other people’s views. Just go read, read a lot, and build your own bedrock, as your own opinion is the most reliable one.