The EPUB (electronic publication) format is an international industry standard designed to allow text display to be optimized and changed for a particular reader. Although it is meant to be device neutral, not all ereaders support EPUB -- Kindle, for example, does not.
Most of the Marx Library's ebooks are provided in pdf format. This is not, however, always the ideal file type to use on an ereader as you cannot adjust the size of the font or always, in the case of the Nook Color, go from page to page easily. A solution for both issues is to convert your pdf to EPUB.
To convert documents to epub, use Calibre -- an open source freely available ebook management program available for Windows, Mac and Linux. Download it here: http://calibre-ebook.com/.