Where's the main documentation for Plone?
Up to Table of ContentsThis FAQ applies to: Any version.
The main starting point is the documentation section on plone.org. The study guide What documentation should I read? provides a brief overview of what documentation you should read - and when - to get you started.
There are several books about Plone. The most up-to-date book is currently Plone Live. It covers most of Plone 2.1, which to a large extent also applies to 2.5.