Documentation
“The source for documentation.”DocBook

The previous edition of the book covered both SGML and XML and was aimed at the DocBook V3.x and DocBook V4.x schemas. It's no longer being actively developed, and has gone out of print, but it is still available online.
Documentation for older releases and several customizations is also available on https://tdg.docbook.org/.
xslTNG Stylesheets

This reference documents all of the parameters, modes, and templates used in the stylesheets. It also provides guidance on how to use the advanced features of the stylesheets.
XSLT 1.0 stylesheets

Wiki
A DocBook wiki is also available. It contains a mixture of pages covering a wide range of schemas and tools. It’s not curated very actively these days and is probably quite out-of-date in some respects.
Other documents
For a time, these documents were hosted on documents.docbook.org which is
now defunct.
Transclusion
“Transclusion” describes the process by which one document incorporates another by reference. In XML, this is most commonly done with XInclude. The presence of IDs and ID/IDREF links in an XML document complicates the transclusion process. Transcluding a document or fragment (for example, a warning or procedure) more than once may introduce multiple elements with the same ID. That, in turn, makes IDREF links to those IDs more complicated.
The transclusion specification describes a relatively light-weight mechanism by which a document author can automatically transform IDs and IDREFs to resolve duplicate IDs and clarify the linking relationships.
- DocBook Transclusion
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The syntax, semantics, and processing model for the DocBook transclusion mechanism.
- Requirements for transclusion in DocBook
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A summary of uses-cases for transclusion of content in DocBook documents.
ISO 690 Bibliographies
Evidence suggests that Jana Dvořáková published a description of how to use DocBook to produce ISO 690 bibliographies. It was lost from the wiki sometime circa 2012, it’s been recovered here as Bibliography acorrding to ISO 690 and ISO 690-2 standards.