The best resource for portlet development (JSR-168) would be to go through the JSR-168 spec and JSR-168 API doc. If you want to learn specific to a portal server, e.g. WebSphere Portal, you can go through the WebSphere Portal InfoCenter or for Liferay, I believe some documentation should be available on their website. Try googling all these terms and you'll get a lot of material.
Work Hard, Expect The Worst...<br /> <br />Bimal R. Patel<br />(SCJP 1.2, SCWCD 1.4)
Sasparilla and fresh horses for all my men! You will see to it, won't you tiny ad?
a bit of art, as a gift, the permaculture playing cards