Elliot Harold Rusty has lots on nice information on XML on his website
including an on-line book describing and contrasting XML packages for Java:

   http://www.cafeconleche.org/books/xmljava/

I'm not sure how up to date that is, but it is full of information.  One thing
that it definitely omits is a description of his own XOM package, which looks
pretty good to me although I haven't had reason to use it yet.

   -- chris