I did search on Google but most of them come up with REST calls for their own site application. I want to see reference documents about rest calls as general manual which I can apply to another web services API myself. Where can I learn about this?
REST isn't anything specific. It's just a word for using HTTP the way it was intended. When you GET a url or POST to one, that is a REST call ;-)
If you haven't yet, take a look at wikipedia http://en.wikipedia.org/wiki/Representational_State_Transfer There isn't much more data on this, as it is a very simple: use HTTP methods the right way There's no specification of the output like in SOAP.