The best source of information about Plone is at www.plone.org. The Plone Book, while not complete, contains a wealth of information about installing and customizing Plone.
Because Plone is built in ZPT, you might need a refresher course in ZPT, TAL and TALES before you can modify things significantly. The Zope Book, an ongoing publication written by Zope Corporation (with much help and comments from the community) has two chapters introducing these intertwined technologies, which you can read at www.zope.org/Documentation/Books/ZopeBook/current/ZPT.stx and www.zope.org/Documentation/Books/ZopeBook/current/AdvZPT.stx.
Zope, including a wealth of documentation about Zope, is available from the Zope community site at www.zope.org.
