DOCBOOK: Referring to external documents

How would you mark up a reference to an external document that isn't 
necessarily a DocBook document?  For example:

	In order to see the sort of trouble you can get into
	by becoming too ambitious in your software projects
	see <whatmarkupgoeshere>Moby Dick</whatmarkupgoeshere>.
	
-- 
Stephen Rasku			E-mail:	
Senior Software Engineer	Web:	http://www.pop-star.net/
TGI Technologies