Hi,
I just read some chapters of the book and I'm really enthusiastic about XMPP now :) Thanks to the author and wrox as well!

Now I'd like to write a web interface for a Java application. There is an extension called Jabber-RPC (XEP-0009) that looks like a nice way to interact with the app.

Does there exist any open JavaScript and Java libraries for for this extension? Or are there other ways to remotly controll an application?

You should be able to write a strophe plugin to implement Jabber-RPC very easily. See Chapter 14 for an example. Another extension to look at is XEP-0050, which is ad-hoc commands. Your Java service would need to expose an XMPP interface in that case, but they are quite powerful.

The XEP-0050 looks interesting.
Thank you!