This week's book giveaways are in the iOS and Features new in Java 8 forums. We're giving away four copies each of Barcodes with iOS: Bringing together the digital and physical worlds and Core Java for the Impatient and have the authors on-line! See this thread and this one for details.
You can usually get by with just using the ejb-jar.xml if you do not want to do anything vendor-specific with your EJBs (or whatever resource you are describing with your DD.) I ran an entire enterprise application using JNDI look ups for the data source and the ejb-jar.xml to define BMPs. The minute you want to migrate to CMP or something that requires tie-in with the container, it will be time to use the jboss.xml or whatever container specific file fits your app server.