Authors don't like it, because they don't get credit. It's the same way with movies and artists/music...when you don't buy their CD's, they are losing money.
I think books where the author has died past the copyright (unless their heirs have decided to renew the copyright), they have posted on sites. Usually classic books. You aren't going to get arrested for reading a book online. You can get in trouble for posting them...and usually they aren't complete, because they will have people who take them down.
There are other ways that are not illegal.
You can buy a book online, people can buy/rent books for their Kindle/or iPod. There are some sites that let you rent books.
