Reviews help both authors and readers. A well written review helps an author promote their work and sell more copies as well as helping readers find the quality literature they're looking for. With the amount of weight put on reviews in the current market, reviews can either make or break an author. If you really want to "thank an author," take the time to write a proper review.
Qualities of a Proper Review:
There are three basic elements to leaving a book review. The more thorough the review, the better. A thorough review justifies the opinion of the reviewer as well as giving prospective readers enough information to decide if a book is worth the purchase.
1) A summary of the book: It's often helpful for prospective readers to read a summary not found in a book's description. Details are intriguing, but spoilers are not. A basic beginning, middle, and end is a good way to go, all of which can be done in a few sentences.
2) Critical opinion of the book: This should include the reviewer's thoughts on plot, characters, story outcome, and the presentation of the book (yes, I'm talking about writing ability, editing, and formatting). What was done well? What did or didn't work? Did the reader feel an emotional connection to the characters and were they sucked into the story? Did the book deliver what was stated in the author's book description and was it worth the time to read? It's important to state a book's positive and less positive aspects, as well as justification in your reasoning for both.
3) Rating: Many sites that allow for positing reviews use a star rating system from 5-1. You can think of this as the American grading system of A, B, C, D, and F. A 5-Star rating (an A) is the best, and 1-Star (an F) is the worst. A 3-Star rating is of course that mediocre place in the middle (a C). Always air on the more favorable star rating. The difference between stars has a huge impact. Honesty is the best policy.
Be Aware of the Following:
1) Books you are unable to complete
Because you hated the story: The majority of books are well put together and have a beginning, middle, and end, but it's true you're not going to like all books. Some topics will rub you the wrong way, turn you off, and get under your skin. At some point in your life you'll put down a book because of this. I certainly have.
Crap editing: Some authors (self-published more often than not) have published stories filled with errors that prevent a reader from understanding the text or following the story, or who have a bunch of text slammed together that actually tells no story, absolutely justifying a 1-Star rating and overall "I hated this book." I've come across many books that fit this description. Click HERE for information on "How Not To Suck At Self Publishing"
These situations can be tricky. On one hand you might want to warn others against purchasing your opinion of lousy entertainment, but on the other hand, the book was the author's (always true!) baby. Consider how your poor rating will affect the author. Reviews and ratings are public. If you can't justify your argument for a poor overall rating, don't bother to leave one.
2) Negativity: Don't be a dick. This should go without saying, but manners aren't a virtue with everyone. Profanity and all-out attacks on an author's skills or storytelling ability are big No-No's! Keep your language PG and your written actions civil.
3) Unfair assessment: The best reviews come from honest strangers, not friends. Even if you're friends with the author, never inflate your opinion to boost the author's mental wellbeing. If you know the author and their book is shit, tell them so and why versus leaving a falsehood for others to interpret at gospel. You can read more about this HERE in "Reviews: Honesty vs. Inflated Truth" Also, if you can't take the time to read an author's work, giving it a chance and your undivided attention, don't review it. Make sure your assessment comes with your attention span.
IN THE END:
Review a book because you loved it and want others to know. Review a book to thank the author for creating a piece of literature that you found entertaining. Review because books are awesome.