appropriate use of entities

I am using a number of entities to split the chapters of a book.

Basically the structure I use is as follows:

a master file that contains:
one entity for each chapter-file: chapter_name => file name
one entity for each part: part_nb => references to contained  
chapter_names

then I have a part file for each part_nb, that contains:
one reference to the part_nb defined in the master file

I choose that structure since I am not yet sure about the final  
position (and number) of all the chapters, and that way I only have  
to modify the master file to update the parts files.

When I use docbook2html (latest Fink version) I get a proper html set  
of files with no specific error message related to references or  
entities, but when I use an app that parses the DocBook based on SAX,  
I get an error when the app reaches the first part. The error says  
that the entity is not defined.

Do I really have to define all the references used in a file within  
that file ? Or are there more flexible ways to do that ?

Jean-Christophe Helary