[BjoernKW]

I'd suggest learning it while you go along. If you know Java SE already Java EE isn't that big a leap conceptually. The difference is really mostly in additional APIs, libraries and tools like application servers (which also for the most part are nothing more than a glorified Tomcat). Those can be learned and Googled as needed.

Moreover, many of the intricacies of enterprise tools like WebSphere AS for instance (which is a pretty notorious example) can't be learned from any book.

Knowing the Spring framework, Hibernate and technologies like JPA is vital, though. Those aren't part of Java EE, however (although they're mostly used in enterprise environments).

Spring.io and Mkyong.com are great resources for getting started with that.