Not all online universities offer an digital library. But, even if yours doesn't, you can access online library materials from many other websites. Online Education Database recently published a list of 250 links to digital libraries and archives. Here's a blurb:

"Hundreds of libraries and archives exist online, from university-supported sites to individual efforts. Each one has something to offer to researchers, students, and teachers. This list contains over 250 libraries and archives that focus mainly on localized, regional, and U.S. history, but it also includes larger collections, eText and eBook repositories, and a short list of directories to help you continue your research efforts."

Bookmark the link and refer to it the next time you're trying to complete an online research project.