Your Legal Rights
When you buy something digital, you don't have all rights to it anymore, than you do, when you buy a physical book. If you took the physical book and photocopied all the pages and uploaded them online, you would be in trouble, for the exact same crime. Each time you upload a book or anything else, you are actually uploading a copy, of that item, and each time someone downloads it, they are making another copy of it. It would be the same as you photocopying a million copies, of the same book and sending them in the mail, to everyone. Nowadays people think things that are easy should be legal, but copyrights are designed, for just these scenarios.
Free with Restrictions
Sometimes some people decide that they want to give away their copyright, so that others can copy the work freely and some even allow their modification as well, but most reserve some rights. The only copyrighted items, that can be used, without any restrictions are called public domain. Copyrights expire, after different periods of time, depending upon media. Books become public domain after 75 years after the author's death or 95 years after writing the book, whatever comes first. Right now, most public domain books include everything written (and translated) before 1920 something. Copyright expiration can and often is continued to extend (which applies to everyone and every creative work), over time, as congress passes new laws.
Everything is Copyrighted
Although many people are able to do many things online for free, what you see online is all copyrighted, at the moment it was created, automatically, according to US law. You do not have to put up a copyright notice to be protected by copyright. There are a whole set of different licenses for allowing people to use your artwork and writing, while still keeping some restrictions on them, in a system called Creative Commons. I make use of this system throughout my website.