Documentation for Python and related libraries falls into a variety of categories. The first that most users encounter is the standard documentation, which can be found in a variety of places.
The standard documentation for the most recent stable release of Python can always be found at http://docs.python.org/. It's easy to search that version using Google; the search box at the top of the page on that site will perform a Google search of only that site. Downloadable versions for printing and interactive use are also made available at that location.
Older versions of the standard documentation are kept available as well, since many users have to maintain code for older versions of the interpreter as well. These can be found at http://www.python.org/doc/versions.html.
The standard documentation is maintained as part of the Python sources, and is released along with Python itself. This should be considered the most definitive documentation on the language definition, the standard library, and the programming interface for the CPython interpreter itself.
There are a range of PythonBooks that have been published in printed forms by a variety of commercial publishers. These range from introductory material on programming using Python as an example language to in-depth treatments of specific topics from a Python programmer's perspective. Look for them at your favorite bookseller, or check them out from your local public or university library.
Alternate Introductory Documentation
Additional introductory and tutorial materials are also available; a list of pointers is maintained at http://www.python.org/doc/Intros.html.
There are a number of documents and other resources available for Python users in languages other than English. These include translations of standard and third-party documents as well as original documents written in other languages. A list of these is maintained at http://www.python.org/doc/NonEnglish.html.
Quick Reference Guides
Sometimes all you need is a hint. Many people have created quick reference guides over the years. The list below was culled from a posting to news:comp.lang.python:
Eclipse SDK Documentation plugin
An easy-to use standard Python documentation plugin for Eclipse SDK. Very handy if you need to get a quick reference from official Python documentation.