Copyright info:

Books can fall out of copyright (into the public domain) in one of 
three ways. First the copyright can expire at the end of it's 
natural term. All books published before 1923 fall into this class. 

Second, books published before 1964 but after 1923 which were not 
properly renewed are considered to also be public domain. 

Finally certain authors can opt to voluntarily give up their copyright 
an donate a book immediately into public domain.

Read more: http://www.gutenberg.org/wiki/gutenberg:copyright_faq