There is a documents page at the DocBookOpenRepository, but most or all of the documents listed there are now also linked to on pages maintained at this Wiki:
DocBookTheDefinitiveGuide: the official documentation for DocBook
DocBookXslStylesheetDocs: including how-to and reference information
DocBookDssslStylesheetDocs: including how-to and reference information
DocBookTutorials: in several languages, including Czech, Spanish, Portuguese, French, and Korean
DocBookHelp: where to get DocBook technical support
DocBookFaqs: various FAQs
DocBookAndXmlCatalogs: how to use DocBook with XML Catalogs
DocBookHistory: the story behind DocBook
Bob Stayton's book: DocBook XSL: The Complete Guide
DocBookXslDocsAuthoring: Guidelines authors/contributors to the DocBookXslStylesheetDocs