The most important category of open-access for books is work that has entered the public domain. In the US, all works published before 1923 have entered the public domain, along with works from later years whose registration was not renewed. Works published in the US from 1923-1963 entered the public domain 28 years after publication unless the copyright registration was renewed. Public domain status depends on national law, and a work may be in the public domain in some countries but not in others. The rules of what is in and out of copyright can be confusing and sometimes almost impossible to determine correctly.
In addition to public domain books that are made available by Project Gutenberg, works digitized by other efforts may be available on an open-access basis. It's not true, however, that any digitized public domain book is also open-access. That's because the digitizer can restrict access to the works using license agreements. For example, JSTOR has many digitized public domain works included in its subscription products, but the terms of the subscription prevent republication of their scans. Similarly, Google puts restrictions on the public domain books from partner libraries that it has scanned, digitized and included in Google Books. While they're available for free, there are limits on what you can do with them.
The public domain is more than just free; it belongs to everyone. Public domain works can be copied, remixed, altered or extended. A book publisher can take a public domain text, print up bound volumes, and sell them in bookstores. A movie producer can create a cinematic dramatization of the public domain work; derivative works such as the movie acquire copyrights of their own and are not in the public domain.