Hello, I am Jamie Robe, author of the Books about Zope
Plone is a Content Management System (CMS) built on top of an advanced software system called Zope.
Zope is an web application development system and server. As a Plone user you have actually been using many of the features of the Zope software without realizing it. For instance, Plone uses the ZMI (actually stands for Zope Management Interface) for the behind the scenes management and construction of websites.
If you are interested in learning more about Zope, I recommend looking at the reviews on amazon.com first. I have a link here to all the Zope books they handle: Zope Books
You can build all sorts of websites and applications just using Zope, although detailed knowledge of it is not required for use of Plone. It is interesting that there is a larger world of Zope users out there who may never even use Plone!
