Reviews help the authors in many ways, not the least of which is letting them know people are reading their works.
Constructive criticism helps an author improve their writing, but be careful. A writer needs to write, but they do not need to publish. Some writers get so discouraged with reviews that they stop publishing - and probably stop listening. Writing is hard work. Always give the writer some kind of praise.
As a writer, I never review a book I don't like. I could name some famous authors whose writing I don't like. The fact that I don't like a book only means it isn't the right author or book for ME. I occasionally note something about a book so that other readers will be aware, but only if I liked the book as a whole, and I always find something good to say about the book and author.
Reviews can be left on most sites where books are sold. You do not have to purchase the book from Amazon to review it, but you will need to be a member. At the end of each of my stories, there is a guest book. Reviews can be left in the comments section. Sometimes, when I'm having an off day, these reviews make all the work worthwhile.
Writing a review can be as simple as "I liked the book." Go on to say why you liked it. I often see reviews that are long and detailed. I rarely read them when I am buying a book. The book description and editor's review gives us most of that information already. Below are some of my favorite reviews on my books.
Linda L. Rigsbee, Author
11/30/08 - About Dear Tales Site:
"Thank you for some very well written stories. I have enjoyed all of them. I don't think that I have read any book that was better written than yours."
8/13/17 - About Nightriders Of The Double Spur
"Very nicely done. I have always enjoyed western stories. Started with Zane Grey as a child in the 50's. You have a nice easy way with a story. Thank you."
2017 - About Once Bitten, Twice Shy
"I will always love this story. I've reread it a million times."
2018 - About Courtship Of The Recluse
"This was one of the most enjoyable reads I've had in many, many books. It was a story that kept true to the character of both Russ & Cindy while carefully kindling and developing a relationship between them. The story moved right along without any boring or dragging episodes, and I couldn't stop reading until Zach finished his lunch. Thank you for sharing this; it blessed me deeply."
4/25/17 - About Savage Wilderness
"I found each paragraph of your writing full of life as it led my mind along the journey as if I was there."
12/15/15 - About Fool's Gold
"I loved this book. After spending so many years trying to get at least one book published, this book has given me some strength to keep publishing. Your writing is amazing, captivating."