I've found this book very helpful. I'd recommend it highly.
Be warned, though, pull the examples from the FTP site,
since there seem to be a lot of corrections in there.

There's some interesting stuff in the earlier examples,
but the last few on different kinds of applications
were the most helpful to me. I grabbed the RMI code from
the mediator/collaborator examples and I used them in
some projects of my own. Very cool!