The Open Knowledge Foundation has published a nifty guide on the basics of Finding Interesting Public Domain Works Online. You can skim the guide in well under ten minutes, and it includes useful links and accompanying descriptions to online collections where PD works can be found, including Europeana, the Internet Archive, and Project Gutenberg. It also contains quite a few references to Creative Commons and succinct explanations of the relevant CC tools, such as the Public Domain Mark and the CC0 Public Domain Dedication. The guide, like all articles at The Public Domain Review, is available for reuse under CC BY.

Bibliotheca Buloviana (Ausschnitt) / Georg Daniel Heumann / Public Domain