I've been using the RI tool to learn EJBs. I'm considering transitioning to JBoss since the RI tool is just for learning. Do you guys think I can benefit by purchasing a book after downloading JBoss or can I use on-line refrences to aid me during my personal devolopment projects?
Once you know one J2EE application server, it's pretty easy to learn another one. Remember that J2EE is a specification, so your code and J2EE-standard descriptors will look the same regardless of your application server.
So, I would suggest learning the J2EE and understanding it well.
The main differences between application servers are: JMS Destination deployment, DataSource deployment, and registering JAAS LoginModules - there aren't any standards for this. Also, the application server-specific deployment descriptors are different between application servers, but they're all trying to do the same thing.
For now, you mitigate change/porting by using XDoclet to generate deployment descriptors. Once EJB 3.0 comes out, you won't need deployment descriptors, so porting will be even easier.
By the way, WebLogic is also a good application server. I've used it in the past and I was pretty happy with it.