For the most part, the REST API is useful for acting on data in an existing mailbox. So that's retrieving data in different formats and importing data (like calendar appointments + contacts).
To handle more administrative functions (and user mailbox functions), check out the SOAP API. Documentation is is found in ZimbraServer/docs/soap*.txt (soap.txt handles user commands, soap-admin-txt administrative commands, etc).
And an FYI: here is a link to the REST API Reference (methods, the different return formats and authentication):
ZCS 6.0:Zimbra REST API Reference - Zimbra :: Wiki
And this thread might be useful as well. It has some pseudo code and a high-level picture of using the SOAP API. Note: when submitting SOAP commands, you can submit in the SOAP XML format or as JSON (and get the return in SOAP XML or JSON). This is described in soap.txt:
http://www.zimbra.com/forums/develop...-examples.html