It really depends if the book is in the public domain. Books such as Harry Potter, Eragon, and audiobooks online The DaVinci Codeare not legally available for free.
The publishers actively search the internet looking for people who have these posted. They can ask them to remove them or even press legal action. Public domain usually includes older or more classic books. A Christmas Carol or The Scarlet Letter would be included.

See the related links below for some free public domain audio book libraries.