There are two main reasons for an author wanting a review:
- good reviews tend to get more buyers of the book; poor ones tend to send buyers away
- generally an author wants to improve his/her writing and constructive reviews can help promote the author's growth as a writer
CharactersCharacters are probably the most important thing in a story. Readers want to feel something about them. There are generally two main characters or groups of characters in fiction - good and bad. Readers want to love the good characters and hate the bad ones. As a reviewer, it is important to let the author know if that was accomplished. Characters also need to be believable, even if they have super powers or wings or some other trait that isn't normal in humans. By believable, I mean that what they do fits their personality and traits. Good characters also tend to have something that they are attempting to overcome within themselves. If this doesn't "fit" into the story or their personality then let the author know so they can improve characterization.
StorySurprisingly enough, the story is not the most important part of the book. Most ideas for stories are pretty good. It is in the way the story is told that succeeds or fails the story. Stories must be told in an interesting manner and it must have something in it to hold the reader's attention. If you're reading something and you are bored before you get passed page three, you should tell the author. Usually the problem isn't with the actual story itself, it is with the way the story is being told. Many authors use a lot of passive sentences instead of active ones. This can make a story very boring. As a reviewer, you need to let the author know this so that he/she can improve this aspect of their storytelling. On the other hand, if an author succeeds in holding your attention to the end of the book, that should also be conveyed so that the author knows that, at least in this area, he/she has succeeded.
Sentences and DescriptionEach sentence should be moving the story forward. Sometimes this is subtle, other times it is direct and obvious. Sentences shouldn't, however, be only descriptive. In other words, a well done story should have a variety of sentence types but should mostly be active. If the author is telling you how someone feels, for instance, it is better if he/she uses active language ( Her stomach lurched and she gagged) than to use something descriptive (She felt sick to her stomach). One evokes feeling, the other doesn't. So if the author drones on and on through the book, without utilizing active sentences, you should let him/her know. This is important, because the author won't improve if you don't help.
Spelling and GrammarI'm not sure why so many authors, particularly new ones, believe that their spelling and grammar do not count in a story. It does. An odd typo or missed error is one thing. A whole book of spelling and grammar errors is quite another. They are distracting, to say the least, and downright foolish on the author's part. So if you read something that is full of these sorts of errors, let the author know. If enough reviews tell the author to smarten up, then perhaps the author will take the hint and learn to use a spell-checker as well as where his/her various grammatical problems lie.
Those are the four main areas that your review should concentrate on. How you put your review together is up to you, but this should give you an idea of what it should contain. Please DO review books. It helps the author, which in turn helps the readers.