In the documentation for manually setting up Boostbook, under "Configuring local DocBook XSL and DTD distributions", the address for the Docbook DTD in the link for the OASIS DocBook site, is http://www.oasis-open.org/docbook/xml/4.2/index.shtml. Perhaps it moved but it should be http://www.oasis-open.org/docbook/xml/4.2/index.1.shtml.