"Open Educational Resources are teaching, learning or research materials that are in the public domain or released with an intellectual property license that allows for free use, adaptation, and distribution."
( "Open Educational Resources | United Nations Educational, Scientific and Cultural Organization." Open Educational Resources | United Nations Educational, Scientific and Cultural Organization. UNESCO. Web. 27 May 2016.)
"GW Open Textbooks" by GW Libraries reused under the CC BY license
Many OER materials are licensed under a Creative Commons License. Creative Commons is an easy way to grant copyright permissions for others to use your creative work.
Depending on the type of license used, you may be able to adapt an open textbook.