В файле /usr/share/xml/docbook/dtd/4.4/dbcentx.mod имеются строки вида:

<!ENTITY % ISOnum PUBLIC
"ISO 8879:1986//ENTITIES Numeric and Special Graphic//EN//XML"
"ent/isonum.ent">

(и им подобные, указующие на другие isoxxx.ent). В то же время сами файлы
(относящиеся к пакету xml-common) называются немного по другому:

[builder@builder SPECS]$ cd /usr/share/xml/docbook/dtd/4.4/ent/
[builder@builder ent]$ ls
catalog       iso-amsc.ent  iso-amsr.ent  iso-cyr2.ent  iso-grk2.ent
iso-lat1.ent  iso-pub.ent
iso-amsa.ent  iso-amsn.ent  iso-box.ent   iso-dia.ent   iso-grk3.ent
iso-lat2.ent  iso-tech.ent
iso-amsb.ent  iso-amso.ent  iso-cyr1.ent  iso-grk1.ent  iso-grk4.ent iso-num.ent
  xmlcatalog

а именно с дефисом после букв "iso". В результате возникают ошибки следующего вида:

docbook2html -d qtiplot.dsl docbook-en/index.docbook -e no-valid
Using catalogs: /etc/sgml/xml-docbook-4.4.cat
Using stylesheet: /usr/src/RPM/BUILD/qtiplot-0.9.5/manual/qtiplot.dsl
Working on: /usr/src/RPM/BUILD/qtiplot-0.9.5/manual/docbook-en/index.docbook
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:308:0:E: cannot find
"ent/isoamsa.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isoamsa.ent",
"/usr/share/sgml/ent/isoamsa.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:312:0:E: cannot find
"ent/isoamsb.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isoamsb.ent",
"/usr/share/sgml/ent/isoamsb.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:316:0:E: cannot find
"ent/isoamsc.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isoamsc.ent",
"/usr/share/sgml/ent/isoamsc.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:320:0:E: cannot find
"ent/isoamsn.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isoamsn.ent",
"/usr/share/sgml/ent/isoamsn.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:324:0:E: cannot find
"ent/isoamso.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isoamso.ent",
"/usr/share/sgml/ent/isoamso.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:328:0:E: cannot find
"ent/isoamsr.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isoamsr.ent",
"/usr/share/sgml/ent/isoamsr.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:332:0:E: cannot find
"ent/isobox.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isobox.ent",
"/usr/share/sgml/ent/isobox.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:336:0:E: cannot find
"ent/isocyr1.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isocyr1.ent",
"/usr/share/sgml/ent/isocyr1.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:340:0:E: cannot find
"ent/isocyr2.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isocyr2.ent",
"/usr/share/sgml/ent/isocyr2.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:344:0:E: cannot find
"ent/isodia.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isodia.ent",
"/usr/share/sgml/ent/isodia.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:348:0:E: cannot find
"ent/isogrk1.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isogrk1.ent",
"/usr/share/sgml/ent/isogrk1.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:352:0:E: cannot find
"ent/isogrk2.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isogrk2.ent",
"/usr/share/sgml/ent/isogrk2.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:356:0:E: cannot find
"ent/isogrk3.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isogrk3.ent",
"/usr/share/sgml/ent/isogrk3.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:360:0:E: cannot find
"ent/isogrk4.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isogrk4.ent",
"/usr/share/sgml/ent/isogrk4.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:364:0:E: cannot find
"ent/isolat1.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isolat1.ent",
"/usr/share/sgml/ent/isolat1.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:368:0:E: cannot find
"ent/isolat2.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isolat2.ent",
"/usr/share/sgml/ent/isolat2.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:372:0:E: cannot find
"ent/isonum.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isonum.ent",
"/usr/share/sgml/ent/isonum.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:376:0:E: cannot find
"ent/isopub.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isopub.ent",
"/usr/share/sgml/ent/isopub.ent"
jade:/usr/share/xml/docbook/dtd/4.4/dbcentx.mod:380:0:E: cannot find
"ent/isotech.ent"; tried "/usr/share/xml/docbook/dtd/4.4/ent/isotech.ent",
"/usr/share/sgml/ent/isotech.ent"

В то же время, если исправить файл /usr/share/xml/docbook/dtd/4.4/dbcentx.mod на
предмет проставления дефисов в нужных местах, то подобные ошибки перестают
появляться.