If you're developing portlets, you probably don't need to concentrate too much on Object Oriented Design (ood). Concentrate on just creating portlets.
What JSR portlet version are you looking at? JSR168 or JSR286. There are a number of good JSR168 books and tutorials on the market, including mine! I have some free tutorials on portlet development too if you follow my signature links.
Most portlet books are available on amazon.com, and you can read reviews there. Also, amazon usually gives a decent discount of the list price.