As I interact with more authors, whether personally or through Goodreads threads, I've noticed a conversational theme which crops up over and over. Reviews: good, bad, ugly, or worse—nonexistent.
Most authors depend on reviews to promote their books to others. Some use them as feedback in order to learn what they might be doing wrong so they know how to improve their writing. Some really strong-willed authors claim to never read reviews, no matter what, because it's not going to change anything they do in the future.
I have issues with those who claim to "never" read reviews, so I'm just going to be honest: I don't believe you. I think you secretly read them and pretend you don't care.
Although most, if not all, authors write for the pleasure of it and the satisfaction involved with the whole creative process, I simply can not believe there are more than a handful of them who truly don't give a rip about whether someone likes and appreciates their efforts or not. If you don't care, then why are you publishing your books at all? Why not write them and put them in a special place in your home where nobody will find them until you're dead and gone? Like the basement freezer; sealed in a ziploc bag & buried the backyard; in your septic tank; in a wall safe behind your mother-in-law's picture. There you go: four perfectly safe, practically-hidden-forever places where your special art can remain concealed, untainted by the eyes of others. Don't thank me for the ideas; just use them. But only if you really, really don't care.
The other 99.8% of those who write creatively do so because they want to share their ideas with the rest of the world. I'm so glad they do, because I need more creativity and imagination in my life. They give me color and nuance in a way I can't come up with on my own. They make me think of things in a totally different way. They make me smile, and they make me cry. And sometimes they make me crazy.
These are the authors who may not live for reviews, but they do thrive on them. One author on a Goodreads thread mentioned that he'd rather have more reviews of all levels than only a few that are all five-star. To leave a book review tells the author you've not only read their book but have taken the time to let them know you appreciated it . . . or didn't. Either way, it tells them you've paid attention somewhere along the way.
I leave reviews for specific reasons. Obviously, if I've enjoyed a book, I want to let the author know. I'm pretty sure most people enjoy being complimented when it's sincere. I'm not a flatterer. If I like you, I'll tell you. If I don't, I'll avoid you but will still be polite if I can't avoid you. I can be tactful if I need to be . . . and uncomfortably blunt, also, as long as I remember to be kind while doing so.
I've left some pretty scathing reviews on Amazon. I've been accused (by someone claiming to not be the author, of course) of being a cheapskate and expecting superb literature for under three dollars. I've been chastised by disgruntled friends of authors for "never" giving good reviews. I've been told to "get a life" by the same not-author who called me cheap. None of those things is true. I just firmly believe in warning book purchasers if a novel is a piece of garbage. It has nothing to do with my personal taste in books, but whether a book is well written, makes sense, and is the best work the author can do.
A newer author will never realize what he or she is in need of learning if readers don't leave reviews. In this era of e-books, sales don't always mean your book is loved by one and all. Someone could download it for free or cheapie-cheap and delete it without finishing, because it was very little investment in their resources. Some sites won't allow an author to promote on them unless a minimum number of reviews are logged on either Amazon, Goodreads, Barnes & Noble, or other places. A good author, whether new or not, needs the encouragement to keep writing.
Read it. Review it. The authors worth their salt will appreciate it. They really do want to know what you think.