Its the metadata for that book. I believe it's created so if you want to import those epubs into a different program, you can do so with all your nice metadata intact.
You can disable this behaviour if you want. Under Prefererences-->Add/Save-->(TAB)Saving Books. There is a checkbox for "Save Metadata in OPF file".
EDIT: At least for epubs, I believe the metadata is also stored within the actual epub, so FOR EPUBS this OPF file might be redundant. But not all ebook formats support saving extensive metadata inline (think TXT, RTF, HTML, etc). I could be wrong about this though, so someone else pipe in.
Last edited by Stinger; 06-01-2010 at 06:48 PM.