Quicklook is a feature of Mac OS X that provides a way to quickly identify a file. This page covers tools to generate Quicklook content.
ePub-quicklook is a QuickLook generator for Mac OS X. It is designed to extract the cover images from ePub files to use as the file icon, and present a nice overview of the ePub in QuickLook. This generator reads the various parameters directly from the ePub contents - so it will work on books that haven't been imported into iTunes.
It is available at: https://github.com/jaketmp/ePub-quicklook