Bibliographic recomendations

For the last 6 months or so I've been writing XSL transforms (typically into
HTML) and I'd say that I've become very good at it. However there are a lot
of things I'm starting to need to do that gets considerably more complex.
Can anyone recomend some intermediate to advanced level XSL books that is
going to be worth purchasing?

Thanks.
> 		Adam van den Hoven
> 		Internet Application Developer
> 		Blue Zone
> 		tel. 604.685.4310
> 		fax. 604.685.4391
> 
> Blue Zone makes you interactive.(tm) http://www.bluezone.net/
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