Here’s a book list I’ve compiled — from the most recent to the oldest — on all of the titles I can find that talk exclusively about DITA XML:
- Practical DITA by Julio Vazquez (October 6, 2009) 112 pages; also available in print or as downloadable PDF from Lulu.com (note that the version from Lulu.com is updated over the first edition which currently is the only version that appears on Amazon.com)
- DITA 101 by Ann Rockley, Charles Cooper and Steve Manning (May 19, 2009) 148 pages; also available in print or as a downloadable PDF from Lulu.com.
- Introduction to DITA: A User Guide to the Darwin Information Typing Architecture-Arbortext Edition by Jennifer Linton and Kylene Bruski (2007) 284 pages
- Introduction to DITA: A User Guide to the Darwin Information Typing Architecture, by Jennifer Linton and Kylene Bruski (April 2006); also available as an “Electronic Edition” (PDF?)
A worthy addition to this list is Louise Kasemeier’s free 76 page PDF DITA for Solo Writers (2007) which looks into the basics of installing and using the DITA Open Toolkit.