This will explain it better than I can:

http://www.google.com/intl/en/press/pressrel/print_publicdomain.html

It's pretty neat. If the book is in the public domain you can read the whole thing. If it's still copyright protected, you can view it's table of contents and some excerpts.

But it's a real nice research tool. As an example, just go to it:

http://print.google.com/

...and enter the word "Disney" and you'll likely find books about Disney you never even knew existed!