Joomla! developer documentation can be found in the following resources:-

• Joomla! 1.5 and later is documented on the wiki at docs.joomla.org.  A good place to start is on the developers page.
• The Joomla! 1.5 codebase has been analysed by an automated tool that looks at specially formatted comments embedded in the source code itself.  The output for the current stable release of the Joomla! 1.5 Framework can be found at api.joomla.org and the current SVN can be found at api.joomla.org/svn.
• Some developer documentation for the legacy 1.0.x series can still be found on help.joomla.org.
• Information on Joomla!'s license, how to license Joomla! extensions, and other licensing issues can be found on the Open Source Matters licensing page.