The document type declaration is also the right place to include other files. The documentation usually tend to become very big, so it's a good idea to split it into several files that are included into a single main file like this:
Example 4-2. Entities used to include other files
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN" [ <!ENTITY introduction SYSTEM "introduction.docbook"> <!ENTITY marketing SYSTEM "marketing-plan.docbook"> <!ENTITY advertising SYSTEM "advertising-campain.docbook"> <!ENTITY conclusion SYSTEM "conclusion.docbook"> <!ENTITY cereals "<productname>Frobozz Cerals</productname>"> <!ENTITY orange-juice "<productname>Hourmade Orange Juice</productname>"> ]> <book id="marketing-study" lang="en"> <bookinfo> <title>Marketing study about &cereals;</title> </bookinfo> &introduction; &marketing; &advertising; &conclusion; </book>
Of course, you must create files named introduction.docbook, marketing-plan.docbook, etc, that will hold your chapters, to make this example work.