Well, I'd humbly recommend the one that I wrote.
JSR 168 Portlet Programming I think Amazon has a pretty heavy discount on the book right now, although that can always change.
The link above takes you to the latest release on amazon. The older release has a number of reviews if you want to see what people think of it. The new edition simply adds some important topics that are hot today, such as creating a
Struts portlet, a
JSF portelt, using AJAX in a JSR168 portlet, and even how do do a basic installation and deployment of a portlet to JetSpeed 2.1 and the Pluto portlet container.
I also provide a bunch of free, multimedia tutorials on doing portlet development, just check out my signature links. The tutorials use IBM's Rational Application Developer (IRAD), but the portlets are all JSR168 compliant, and it's the concepts that are important, not the development tool.
Good luck on your portal quest!
-Cameron McKenzie