Below are resources relating to the creation of OER. Whether you're adapting an open textbook or developing a new resource, you'll want to find open content (images, videos, text) that is available for use.
A Guide to Making Open Textbooks with Students (2017) - Designed to offer guidance for those interested in involving students in the process of open textbook creation and modification.
Guide to Developing Open Textbooks (2016) - Developed "to assist teachers and educational technology administrators to build, manage and maintain an open textbook platform."
Open Pedagogy Notebook - A site created by Robin DeRosa and Rajiv Jhangiani, to facilitate online conversations between educators.
6 Steps to Adapting an Open Textbook - A post that clarifies some issues in adapting an OER textbook.
Localization - A module designed to expolre this aspect of OER, "the process of taking educational resources developed for one context and adapting them for other contexts."
Opening the Textbook: Open Education Resources in U.S. Higher Education - A 2017 Survey Report conducted by Babson Research Group, on the state of OER in the United States.