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 4.1 docs are available at http://docs.atlassian.com/software/jira/docs/api/4.1/. For previous versions, substitute the appropriate version in the URL.
You can also view a JDiff report of API doc changes between JIRA 4.0 and 4.1 here: http://docs.atlassian.com/jira/4.1/jdiff/changes.html