CiviCRM is accompanied by an up to date and ever growing range of documentation.
For users and administrators we have:
For developers we have a set of Developer Documentation which covers everything you need to know to extend and develop with CiviCRM.
Our wiki also hosts documents that support current projects and work in progress, such as requirements, specifications, project plans etc. That is all found here: http://wiki.civicrm.org/confluence/display/CRM/
Anyone can edit the book and the wiki! Reviewing and improving documentation is a great way for non technical people to contribute to CiviCRM and is also a great way to improve your understanding of CiviCRM. See our improving documentation page for more details.