DITA Publications
Books

Introduction to DITA, by Jennifer Linton and Kylene Bruski (Comtech Services), 2006). In 2007 JoAnn Hackos published the Arbortext Edition of her Introduction to DITA.

DITA Pocket Guide, by SiberLogic, 2006.

DITA Open Toolkit User Guide 1.3.1 (PDF),
HTML Frameset version
by Anna van Raaphorst and Richard H. (Dick) Johnson,
VR Communications, 2006.

Developing Quality Technological Information: A Handbook for Writers and Editors (2nd Edition) , by Gretchen Hargis, Michelle Carey, Ann Kilty Hernandez, Polly Hughes, Deirdre Longo, Shannon Rouiller, Elizabeth Wilde (IBM Press, Information Management Series, 2004).

Information Development: Managing Your Documentation Projects, Portfolio, and People, by JoAnn Hackos (Wiley, 2006).

XML and Framemaker, by Kay Ethier (Apress, 2004).

Articles
DITA - Getting Started by Christian Kravogel and Boris Horner
Going DITA by Constantine Hondros
To DITA or not to DITA? - Palimpsest, Friday, January 06, 2006
DITA for DocBook - Implementing the Darwin Information Typing Architecture for DocBook.
Planning for DITA Success - Part One: How to Set Up the Right Team and the Right Strategy (PDF) by the Rockley Group
Planning for DITA Success - Part Two: How to Deploy DITA, Step-By-Step (PDF) by the Rockley Group
DocZone is doing it with DITA by the Rockley Group
DITA - What you need to know (PDF) by the Rockley Group
Subsetting and customizing DITA by Bernard Aschwanden
To DITA or not to DITA? by Scriptorium Palimpsest
DITA and the Beatles by Bill Trippe
DITA Tools from A to Z, by Bob Doyle (from special DITA issue of STC Intercom)
Hidden Cost of DITA, by Sarah O'Keefe (from special DITA issue of STC Intercom)

Articles at IBM developerWorks

Migrating HTML to DITA, Part 1: Simple steps to move from HTML to DITA
Migrating HTML to DITA, Part 2: Extend the migration for more robust results
 

Online Tutorials and Workshops