Well, writers do. If you regularly read e-books, consider reviewing them online at places like Amazon.com and Goodreads.
You see, independently published writers don't have access to national venues - and seldom to local ones - that feature book reviews. Word of mouth, or word of computer, is vital to them.
I'm not saying you have to love every book you review. Far from it. Be honest. But consider taking five minutes to write a brief "Loved it/Liked it/Tolerated it/Hated it" kind of notice.
Nothing elaborate is necessary; the goal is to let readers know what you found good, indifferent, or bad without giving away major plot points (spoilers) or missing the point of the book entirely.
What? Do such things happen?
Certainly. If Oedipus the King was a novel, you'd probably find someone who on Amazon.com had written, "The big surprize at the end of this book is that the murdrer is really oedpipus him self." (If you think I'm exaggerating the subliterate quality, go read a few reviews there). Don't give away the ending, please!
Or you might find a review of, say, Lady Chatterley's Lover that misses the point: "Some interesting observations on gardening and game-keeping in England are lost when the author wanders off into a love story."
You can do better than either of those. I encourage you to do it. Other readers will appreciate it, you'll have a sense of accomplishment, and writers will be pleased that someone is reading and reacting to their work.