Open Access can be any material that is freely accessible. This includes material out of copyright, but also material that has been published as open access. Material published as Open Access can still be under copyright and cannot be edited without permission.
Documents published by the Federal Government and unclassified, are generally not under copyright. The one exception are publications that contain articles, reports, etc. that are copyrighted.
There are all kinds of government documents on almost any subject imaginable, from books to coloring pages for kids. The Ozuna Library is a selective Federal Depository Library which means that we automatically receive a select collection of government publications in print and electronic format. They can be found in Palomino Search.
Additional documents, websites, and videos can be found at
Not free and technically not open - but just to show that IF you really cannot find the text book you need or want, there are lower cost options:
The sites below offer a variety of freely available material and is just a sampling. If you find a great site, let me know!