xmllint validation error

Hi,

I tried to validate my xml document (set), but I get the following error:

$ xmllint --noout --noent --valid pyblio.xml
pyblio.xml:35: validity error: Element book content does not follow the
DTD
Expecting ((title , subtitle? , titleabbrev?)? , bookinfo? , (dedication
| toc | lot | glossary | bibliography | preface | chapter | reference |
part | article | appendix | index | setindex | colophon)*), got
(bookinfo indexterm indexterm preface chapter chapter chapter chapter
chapter chapter reference appendix )
</book>
       ^

 I've tried some modifications already, but now I have no idea what to
change, what to try.
Any suggestions?

Thanks
Z. Kota