It's only complicated in the US; it's pretty straightforward elsewhere.
In the US, stick to books published before 1923, and you're completely "safe". It gets complicated for things published after that - that's when you get into issues of copyright renewals, and all that kind of thing. Prior to 1923, though, and it's guaranteed to be in the public domain (in the US).
This might help to clarify it for you:
http://www.copyright.cornell.edu/tra...lic_Domain.htm