Hussein will probably answer this first, and more completely, but you need to add the docbook element "preface" to your XXE docbook configuration ($XXE/config/docbook/docbook.xxe).
Look for the list of <rootElementLocalName>xxx</rootElementLocalName> lines; you should see part, chapter, appendix, and some others. Just add preface to that and restart XXE and you should be good. @alex
