DOCBOOK: Any suggestions for a GOOD XML reference?

Hello All,

I am learning XML while using the docbook DTD and really need a good 
reference. For example, I have "DocBook: The Definitive Guide" and "XML 
Bible" that do not provide information about the following DTD constructs:

<!ENTITY % local.list.class "">

When I go through the various modules in the docbook DTD I cannot figure 
out where "class" comes from or what "local.list.class" actually means.

Can someone please provide what they consider a good reference that 
explains what is going on in the docbook (XML programming in general)?

I would like to either alter the DTD for my purposes or suggest a change 
intelligently but I can't until I understand what's going on.

I did perform a search through the mail list but couldn't find 
information about this. This shouldn't surprise anyone because I am not 
sure what to search for.

Thanks in advance.

Jeff Biss