I've just gotten Robert Bill's book "Jython for Java Programmers" and I really like it. I've been using Jython for about a year and a half but I've never made an exhaustive study of it and it's never been my primary language. I use it to accomplish lots of smallish tasks, most of which are related to zxJDBC (full disclosure - Brian Zimmer is a former colleague of mine).
As a result of this haphazard learning curve, I don't always know what is available. I've been scanning the first few chapters for about half an hour, and every page seems to hit me with an "I didn't know you could do THAT" feeling. "If only I knew THAT, such-and-such a task would have been a breeze". For example, I never knew you could do
>>> import java
and see a list of method names for that interface. That is POWERFUL. Up to now I've had to scan javadoc pages and I've done so on hundreds of occasions while I was in jython
Anyway, if you've been using Jython for awhile, but not intensively, this is a good book filling in the gaps in your knowlege.