DocBook 5 as Normative Reference? Uncited

1.2 Normative References reads in part:

[DocBook 5: TDG] Norman Walsh. DocBook 5.0: The Definitive Guide. O'Reilly Media. April 2010.

Aside from the question of citing non-normative material as normative, this reference isn't used in the draft.

The one possible reference, under Appendix A 1:

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Published specification:

This media type registration is for DocBook documents as described by [DocBook 5: TDG].
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In fact has a URL that points to Docbook 5.1.

http://docs.oasis-open.org/docbook/docbook/v5.1/csprd01/docbook-v5.1-csprd01.html#bib.docbooktdg5

That being the case, the reference with the label DocBook 5 can be dropped.