Liferay's official documentation is located in the Portal Administrator's Guide, which you can download from http://www.liferay.com/documentation/liferay-portal for free. You can alternatively purchase a paper version.
Other sources of documentation for Liferay include the Wiki and our Forums. Both of these are maintained by a combination of Liferay staff and community.
Community Involvement #
Liferay strongly encourages users to get involved in the community. Part of that involves maintaining the documentation (this wiki). This page links to pages with further information about the Wiki and other forms of documentation, including third-party books.
For information on translating Liferay, see Translating Liferay to a New Language