Did you read the documentation? It is very good, and includes a portlet primer. http://docs.jboss.com/jbportal/v2.6.4/referenceGuide/html/tutorials.html
For general portlet programming, there is a free pdf available at http://www.manning.com/hepper/. It uses a different portal, but most of the portlet-related stuff applies to all portals.
Even the portal spec is very readable: http://jcp.org/en/jsr/detail?id=168
I also have another reference available, specific to JBoss Portal, but it is not free. I can supply the URL if you like.