One thing you will hear from time-to-time is that author's live and die by their reviews. I don't think truer words have ever been spoken. One review certainly won't make or break a book, but the most powerful marketing tool ever devised is still word-of-mouth. Especially in this day of Indie and Small Press authors, it is important to let a book's potential audience know what you think. Now you don't need a degree in English or journalism to write a review. You don't need to be on staff at a magazine or newspaper. Most online bookstores like Barnes and Noble, BAM, Amazon, iBooks, Copia, Kobo, etc. have a place with each book that the general audience can write a review. There are also sites such as Goodreads whose whole reason for existing is to bring authors and readers together. Goodreads is the one social networking site for writers and readers I know of. I'm sure there are others. If you know of any, just put them in the comment field below.
"Yes, Kurt," you say, "but how do you write a review?"
Most of these sites have a star rating system they start at one star for bad and go up to five stars for really really good. This is a good place to start. Note, most places will let you change your mind. So the first thing I think of is, "How much do I like this book." This is a technique I learned doing quality control in graphic arts. Look at the overall piece and give an overall impression.
There are a couple of ways to go from here, but remember, you will have to justify your impression at some point.
Some people like to start off with a synopsis. In other words, without dropping spoilers, what is the story about? As an adjunct professor, students ask me about writing a synopsis frequently. In three to five sentences, you need to describe the conflict and goals of the protagonist and antagonist. A basic format to follow is:
1. Who are the protagonist and antagonist?
2. What do they want to achieve by the end of the story?
3. What is that first crisis that sets the protagonist and antagonist in collision?
4. You can through in a few details such as love interests and reversals if you think it adds to the synopsis without giving the story away.
Not every review I've seen on Amazon includes a synopsis, but it can be helpful in letting a potential reader know if the story might excite them.
Now comes the hard part: justifying your rating. You gave three stars (for the sake of argument). What kept it from being four? What saved it from being two? The easiest thing to do is ask yourself "What was good about it?" Did you like the characters? Could you identify with them? How was the dialogue? Many otherwise great stories come apart because of dialogue. Is it believable? Is it to awkward or stiff. Is it too formal or academic. Conversely does it flow naturally. Was the story line believable, in the context of the genre? This can be difficult in the cases of science fiction or fantasy. The characters are able to do things we can't do in our world. In the cases of these two genre, a better question is, "Did the author help me to suspend my disbelief." Suspending disbelief means that the author has sucked you into the story before you have a chance to question the facts and validity of how they make things work. However, sometimes the author does something that bounces the reader out of that accepting mode and has them looking around, wondering how they got to this point.
A good example for me is Indiana Jones and the Crystal Skull. I liked Raiders of the Lost Ark and The Last Crusade. The Crystal Skull was far better than the Temple of Doom. Where did it fall apart? For me, it was the introduction of the aliens. Don't get me wrong, I have an open mind about extraterrestrials and whether or not they visit Earth. My problem was the cliché way they were handled. The old, alien standby, the Grays, along with other things that have appeared before. I think the writers got lazy and were relying on special effects here. Everything that the characters did up to that point just fell apart. My suspended disbelief came crashing to earth.
Another thing to look at is spelling and grammar. Were their any typos that really made you stub your toe? You don't have to go digging for them. I know of many good works were there are a few typos sprinkled in. In most cases, if the writing is good, they're easy to miss. I'm talking, are there many mistakes that keep jarring you out of the flow of the story.
Now some etiquette.
"What do I do if I just don't like a book?" As painful as it may be, honesty is the best policy. That being said, there is no reason to be mean. My policy it that, if a book falls below three stars, I have to make a hard shift to constructive criticism. Try really hard to find something I like and start with that. It is always best to start with the positive. Indie authors have one advantage over traditionally published authors. It's easier to correct mistakes if you're self-published. I had one person come back to me after I reviewed a book that had fallen short and asked me to review the updated version. Vast improvement.
"What if it's just not something I normally read?" In many cases, it's is best to say so from the get go. If the book is an exception and you really liked it, then the author should be flattered. If it was what you expected, you can say something like "People who usually read this type of story will probably like it." I'd try to be a little more diplomatic, but the author can't expect a glowing review if you don't even like the genre. If you do like a story that is otherwise in a genre you don't read, it is a pleasant surprise. This is also a case where you can consider punting and not leaving a review.
One other point: If you didn't read it don't review it. Yes, you may find the cover or title problematic, but as the old saying goes, "Never judge a book by its cover." If you think the cover or title are objectionable, then read the book before making the final judgment. If you find it surprisingly good, the author may appreciate knowing that the title or cover aren't having the desired effect.
That is pretty much how I do it now. It is a process that evolved over time. You may want to add things or drop some of the things I do. I don't pretend my way is the only, or even best, way to review a story. I merely hope that it provides a good starting point for the would-be casual reviewer.