Documentation for JIRA 4.2. Documentation for other versions of JIRA is available too.
The JIRA API docs are available online. They are most useful for:
The latest API docs are available at http://docs.atlassian.com/software/jira/docs/api/latest/. The 4.2 docs are available at http://docs.atlassian.com/software/jira/docs/api/4.2/. JDiff reports listing the changes to the JIRA API between releases are available here. For previous versions, substitute the appropriate version in the URL.