DITA in a nutshell:

A high-level intro:
* http://www.oreillynet.com/xml/blog/2005/10/going_dita.html

A short case study/interview:
* http://thecontentwrangler.com/article/astoria_softwares_move_to_dita_an_...

The new DITA 1.1 architectural spec has a lot of decent intro material, and is up to date:
* http://dita.xml.org/standard

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