Split into multiple docbook files

The best way to handle big documentation is to break it into multiple files. Docbook allows you to do that by using XInclude. Here is how it is done.

<?xml version="1.0" encoding="UTF-8"?>
<!-- Here is the master file called book.xml -->
<!-- Note the word 'book' after DOCTYPE below.-->
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" 
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<book>
  <title>My Docbook Title</title>
  <!-- Include intro chapter from external file, intro.xml -->
  <xi:include href="intro.xml" xmlns:xi="http://www.w3.org/2001/XInclude" />
</book>
<?xml version="1.0" encoding="UTF-8"?>
<!-- Here is the intro.xml file to be include inside book.xml -->
<!-- Note the word 'chapter' after DOCTYPE below.-->
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" 
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<chapter id="intro">
    <title>Intro</title>
    <para>Imported chapter.</para>
</chapter>

To validate the files, you can use xmllint --noout --xinclude --postvalid --noent book.xml. To create the PDF for your whole document, run

xsltproc --xinclude /path/to/your/docbook-xsl/fo/docbook.xsl book.xml > book.fo
fop -fo book.fo -pdf book.pdf

For more details, see http://www.sagehill.net/docbookxsl/ModularDoc.html.