In an effort not to reinvent the wheel (or in this case a research guide), here are a few additional research guides that provide additional information and resources.
What are they
Things to Ponder
How are you defining OER?
All OERs are not created equal.
Evaluation is a must!
Need a Game Plan
It won't be easy!
Here are a few sites that offer free and/or low cost textbooks. Many are international in scope. Quality of resources do vary, so make sure you review the titles thoroughly.
Examples of open education resources, there are many more.
Examples of open education courses, there are many more.
Not a complete list.