DOCBOOK: [DocBook] Breaking down into separate files

Hi all,
I'm just getting up to speed with DocBook (via NWalsh's The Def Guide).

I can produce my document in rtf and html ok, but it is huge - and I want to
break it up.  I've found how to insert it into my master as an entity, but
since the "child" sub-file can't contain a DTG I can't see how to get
emacs/psgml to allow me to process the DTD or allow my to insert elements.
I've set the sgml-parent-file parameter, hoping that this would make emacs
pull the DTD from the parent file, but this didn't happen.

Can anybody give me some direction in this please?

thanks,

Ian