Chapter as a separate file

docbook-example01.sgml

<!DOCTYPE book PUBLIC"-//OASIS//DTD DocBook V4.1//EN"[ 
<!ENTITY chapter1 SYSTEM "chapter01.sgml"> <!-- We include this file and use it --> 
<!ENTITY  lugname "Bergen Linux User Group"> <!-- This string we use a lot --> 
]> 
<book> 
<title>Outlining a DocBook Project</title> 
&chapter1; 
</book>

chapter01.sgml

<chapter id="introduction"> 
<title> 
Introduction 
</title> 
<para> 
With DocBook, your world gets clearer. <emphasis>&lugname;</emphasis> leads the way. 
</para> 
</chapter>

The result: example01-plain.html or example01-cooked.html