DocBook is almost 10 years old. It began in 1991 as a joint project of HaL Computer Systems and O'Reilly. Its popularity grew, and eventually it spawned its own maintainance organization, the Davenport Group. In mid-1998, it became a Technical Committee (TC) of the Organization for the Advancement of Structured Information Standards (OASIS).

Oreilly Free ebook : DocBox the Definitive Guide

A key feature of SGML and XML markup is that you validate it. The DocBook DTD is a precise description of valid nesting, the order of elements, and their content. All DocBook documents must conform to this description or they are not DocBook documents (by definition).

A validating parser is a program that can read the DTD and a particular document and determine whether the exact nesting and order of elements in the document is valid according to the DTD.

If you are not using a structured editor that can enforce the markup as you type, validation with an external parser is a particularly important step in the document creation process. You cannot expect to get rational results from subsequent processing (such as document publishing) if your documents are not valid.

The most popular free SGML parser is SP by James Clark, available at http://www.jclark.com/.

SP includes nsgmls, a fast command-line parser. In the world of free validating XML parsers, IBM AlphaWorks's xml4j and James Clark's xp are popular choices.