The best information about Plone is available at www.plone.org, the home site of the Plone Project. The site contains extensive documentation and configuration examples, although as with many open-source products, the quality of the writing is uneven at times. Perhaps the most famous Plone site is ZopeZen (www.zopezen.org), which has frequent discussions and news reports on Zope, Plone and Python.
If you want to learn more about the CMF, visit cmf.zope.org. Documentation there is somewhat sparse, but it also is aimed at the more serious and experienced Zope developer than the Plone documentation.
Information about Zope itself is at www.zope.org, the Zope community site.