The Copyright Act (Title 17 of the US Code) gives authors or creators of original works the exclusive right to:
copy the work;
distribute the work;
display or perform the work publicly; and
create derivative works from the original work.
Copyright applies to any work that is “fixed in any tangible medium of expression”, including books, of course, but also photographs, drawings, music, architecture, drama, sculpture, web pages software, and multimedia works. No copyright mark or registration is required for copyright to apply, so almost all modern works are under copyright. That means that you can’t copy, distribute, display, or create derivative works from them without the explicit permission of the copyright holder.