It has been my experience that there are a few really awesome gifts you can give an author: a great thesaurus, a year's supply of coffee, but the absolute best gift you can give an author costs nothing but a few minutes of time: writing a quick review of their book.
Speaking as an author myself, I can personally tell you that it is not only time consuming to promote a book you've written- time that is taken away from actually writing, and this isn't JUST indie or self published authors- NY Times bestsellers also have to devote time to spreading the word about their books; but it can also become rather costly, too. Developing graphics to help catch a readers eye, paying for promotional items or tours or ads... it all adds up.
Reviews are read by others curious about a potential book purchase. If they read how much a person loved or loathed a book, it really does make a difference in whether or not they take a chance on it themselves. Reviews are also crucial to authors who want to hone their craft. I have yet to meet an author who doesn't have a legitimate desire to give readers the very best story they can; and reviews help authors do that. It's like the line in the movie Jerry Maguire- "Help me help you."
So, should you wish to say "Dear Author, thank you for giving me a quick vacation, for giving me yet another world to escape to or another fictional boyfriend/girlfriend to fall in love with. Thank you for seeing me through a difficult time," you can do that via a review.
It only takes a few minutes to let an author, and the world, know how you felt about a book. If you be so kind to consider sharing your thoughts on a book, ANY author's book, please do so. Grumpy Cat memes and someecards will still be there when your finished :-)
On behalf of every author I know, I give you my thanks!