If you ever told me, "I loved your book!" you made my day. That book (any of them) was my baby, and putting it out there for everyone to read was both exciting and terrifying. Your compliment gave me the best warm-fuzzy ever! Even if you thought it was just okay, I'm glad you liked something about it. If you thought it was a disgusting piece of garbage and a waste of paper, I will kindly ignore your bad taste because I'm obviously nicer than you.
But do you know what's worse than a luke-warm review? No feedback at all. Disappearing into obscurity. Horrors! All those months of creative effort, and NO ONE NOTICED! It's an author's nightmare.
So if you liked my book, you can do one more thing for me, and for other authors whose books you've loved: tell others.
Post it online. If you buy on Amazon or another online retailer, you can leave a review. It's easy. "I liked this book" is sufficient. Or you can ramble on for paragraphs, listing everything you loved or hated about the characters, the setting, the author's voice, or the name of the hero's dog. Why bother to do it? Some readers pour over the reviews as a guide when deciding to buy. Others just look to see how many stars a book has been given by readers, which is prominently displayed beneath the title on Amazon. It affects sales. (Did you think this was just about adoration? No, I'm a crass materialist and I like paychecks.)
If you're really into reading, try Goodreads or similar sites. You'll find people who like the same things you like and their reviews might lead you to other terrific authors to try. Or you can warn them about a book that disappointed you. Go ahead, vent. It's anonymous and free. Just for God's sake have an opinion!
Authors will adore you for it. We really, really will. We're often shy, introverted types who keep our crazy ideas to ourselves, but on the inside we're attention seekers saying, "Here's my idea of a good story! What do you think?" We want you to tell us.
(Personal plug of last book because hey, this is my blog, and it was great!)