A repository is a collection of the actual files for Open Educational resources such as textbooks (as opposed to sites that link to OER files). Some books are stored as a single PDF file, but others may be a collection of files with each file containing a chapter, a learning module, an exercise, etc. When stored this way, it is easier to use and edit a part of the book.
Many repositories are set up and maintained by colleges and universities and made accessible via the web. Each repository has a different web interface. Most repositories also include learning modules that are smaller or less comprehensive than a textbook - such as presentations, individual assignments and readings.
Many open works are stored in more than one repository. Among those, one repository may contain the work in only one format, say a single .pdf file, while another repository may have the same work in many separate files.
You should note the distinction between lists and repositories. There is probably never going to be a repository with all the books on it. For example, more than half of the books listed by College Open Textbooks (COT) are on single author sites, not large repositories.
Most repositories want to promote the books they have and they have rules that exclude many books. For example CNX and Global Text Project only accept CC BY. The COT list has 550 books aimed at community colleges. MERLOT and USG Share have more textbooks (upwards of 1000) because they include upper division texts.
It is hoped that in the near future, an OER search engine will be created to help users find materials in multiple locations.
Here are a few examples of repositories for textbooks, courses and other free resources that are good starting points.
||ADDITIONAL SOURCES - including free but not all open resources, and materials for K-12|
||Textbook Media (some open textbooks)
There are many schools and institutions offering courseware via Apple's iTunes U (audio & video podcasts of lectures tec and also documents for the course)
Lulu (has some open textbooks)
Following is an ebook that Cowles Library owns on the topic. You will need to authenticate with your Drake ID# and password to access.
Creative Commons works to free licenses and tools that anyone can use to share their educational materials with the world. Their licenses make textbooks, courses, and lesson plans easy to find, easy to share, and easy to customize and combine — helping to realize the full benefits of digital and print educational resources.