Open textbook authoring, editing, and adapting is a quickly changing landscape. Here are a few guides and groups I've found that engage this topic.
VTechWorks is the Virginia Tech institutional repository, managed by the University Libraries. Its purpose is to highlight, preserve, and provide unrestricted access to the work of faculty, staff, and students, as well as the intellectual output of the university in its land-grant mission to serve the Commonwealth of Virginia, the nation, and the world community through the discovery and dissemination of new knowledge. New user registration Questions? Contact Philip Young.
Open Textbook Library is a project of the Open Textbook Network and the Center for Open Education at the University of Minnesota. If you've created (or found) an openly licensed, complete, and downloadable textbook whose license allows adaptation or derivatives, please consider submitting it to be listed in the Open Textbook Library. Contribute material
MERLOT is a curated collection of free and open online teaching, learning, and faculty development services contributed and used by an international education community. MERLOT is a program of the California State University System partnering with education institutions, professional societies, and industry. New user registration Contribute material
OpenStax CNX "Connexions" is a free, open platform developed at Rice University for development, editing, and mashup of openly licensed content. The content in OpenStax CNX comes in two formats: modules, which are like small "knowledge chunks," and collections, which are groups of modules structured into books or course notes, or for other uses. New user registration Login to contribute material
There are many other disciplinary and institutional repositories and "referatories" which aim to collect, curate, and enable ease in finding openly licensed resources. Ask your disciplinary societies if such a repository exists for your subject area.
You will also want to make certain that you mark your work so that everyone knows up front about the intelletual property status of your work. Include author and license information, and make sure that the license on your work is machine readable (easy; technical). This will enable search engines with "usage rights" filters to find things you create. (For example: Google Advanced Search offers filtering by "usage rights".)