Since Designer has only had one major upgrade since the books Mike mentioned were published, they still contain useful information. Other than that, the best documentation is the help pages that come with the product. The Program Group on your start menu will have a shortcut to that once you've installed the Designer client.
Another place to look is the papers that the Oracle Development Tools Users Group (ODTUG) keeps in its online library. You can get access to these by becoming an Associate member for free - though we encourage you to become a full paid member. Go to http://www.odtug.com/apex/f?p=500:800:0::::: to join and then http://www.odtug.com/apex/f?p=500:565:0:::565,RP:P565_TOPIC_ID:9 to see papers on Oracle Designer.
You are right. I am using this tool since last 3/4 months. The help pages that come with the product are good if you stuck somewhere not for a beginner who need a systematic approach to understand the flow.