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ISBN 1-56592-580-7
Doc Book: The Definitive Guide is the complete and official documentation of the Doc Book Document Type Definition (DTD) and many of its associated tools.
Doc Book is a system for writing structured documents using SGML and XML. It provides all the elements you'll need for technical documents of all kinds. A number of computer companies use Doc Book for their documentation, as do several Open Source documentation groups, including the Linux Documentation Project (LDP). With the consistent use of Doc Book, these groups can readily share and exchange information. With an XML-enabled browser, Doc Book documents are as accessible on the Web as in print.
Doc Book: The Definitive Guide was written by Norman Walsh, the author of the XML implementation of the Doc Book DTD; and Leonard Muellner, the manager of O'Reilly & Associates' Production Tools Group.
Also, O'Reilly has made "DocBook: the Definitive Guide" available as part of their Open Books Project: www.oreilly.com 
There's an updated version of the O'Reilly Book on-line at docbook.org where several different versions of the book are detailed.
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