There's no need to put together course packs that students have to pay for if the library already has access to the material. Before you adopt a resource that students have to pay for, please check the library catalog or ask your liaison. If we have access to it, you can link to it from Blackboard.
The Public Domain refers to works that were created in certain time periods that are no longer protected by copyright, and can therefore be shared and used in your class. If a work is in the public domain, you are free to use that work in any way that you choose – digitize it, re-publish it, post it on the web etc. with appropriate attribution.
* For more info, please see Cornell's "Copyright Term and the Public Domain in the United States."
Tools for determining fair use and public domain status:
Good sources of content in the public domain: