Hi ksterling, welcome!
If, by public domain books, you mean those from Project Gutenberg, and you want to convert text to prc, then you can do so using Mobipocket Creator (please see link below). It's free. It also outputs an html file, and it can also convert html, MS Word, and Adobe PDF documents to prc.
Here is where you can download Mobipocket Creator. Be sure to download also the Mobipocket Reader which can be found on the same page:
http://www.mobipocket.com/en/Downloa...sp?Language=EN