$ cat my.docbook 
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.1//EN">

<book>

<bookinfo>
<title>Hello, World!</title>
</bookinfo>

<chapter>
<title>First title</title>

<para>This is my first DocBook paragraph.</para>

</chapter>
</book>
$ docbook2html my.docbook 
Using catalogs: /etc/sgml/xml-docbook-4.1.2.cat
Using stylesheet: /usr/share/sgml/docbook/utils-0.6.14/docbook-utils.dsl#html
Working on: /tmp/db/my.docbook
jade:/tmp/db/my.docbook:2:54:W: cannot generate system identifier for public text "-//OASIS//DTD DocBook V4.1//EN"
jade:/tmp/db/my.docbook:2:54:E: reference to entity "book" for which no system identifier could be generated
jade:/tmp/db/my.docbook:2:0: entity was defined here