Wouter Van Vugt's book "Open XML Explained" is now available for free download from OpenXMLDeveloper.org. The book is based on the content used in the Open XML developer workshops (available here), with several additional topics included as well. At 128 pages, this book is essentially a high-level tour of the features of Open XML, with low-level details provided for features that most developers will want to use in typical applications.
For now the book is only available in soft copy format, but we're working out the details of how to make it available in hard copy as well. Meanwhile, I have a few copies that I'll mail to anyone who sends me an email with a how-to article for OpenXMLDeveloper.org.
Your email should include a code sample (in any language) that shows how to read or write an Open XML document or data contained within a document, and a brief writeup of how it works. Your sample shouldn't duplicate an existing article on OpenXMLDeveloper, so check out the articles already published on the site to be sure. Also include the mailing address for your book, and the size of t-shirt you wear.
I'll get your article published on OpenXMLDeveloper, and you'll receive a book and a t-shirt for your efforts. I have a few dozen copies of both (and yes, I'm still mailing out t-shirts if you're waiting on some from a recent workshop), so this will be on a first-come basis. If your sample is in a language we've not yet seen on OpenXMLDeveloper, I'll throw in a couple extra books as well.