PURLs (Persistent Uniform Resource Locators) are Web addresses that act as permanent identifiers in the face of a dynamic and changing Web infrastructure. Instead of resolving directly to Web resources, PURLs provide a level of indirection that allows the underlying Web addresses of resources to change over time without negatively affecting systems that depend on them. This capability provides continuity of references to network resources that may migrate from machine to machine for business, social or technical reasons.

via PURL Home Page.

I don’t know what any of that means but purl.org was a nameservice in the .epub XML file content.opf.  Purl.org represented some meta data which the nook never did understand anyway.