The Ecma TC45 working group has released an updated version of the draft spec. You can download it here, as a set of five documents in either PDF or DOCX format. There is also some information about this latest revision of the working draft on Brian Jones's blog.

This version of the spec is the most polished yet, and it has been significantly re-organized to make it easy to use. There are many new charts, diagrams and figures inserted, as well as entirely new sections covering details that hadn't been included in prior versions. There are quite a few how-to samples in the spec now, which will be valuable to developers.

The spec is no longer a giant monolithic 4000-page document. Instead, it has been divded into five distinct documents, which are briefly described below. Note that the link on each section takes you directly to the PDF version of that document. (Note that the DOCX files use the latest changes to the schemas and therefore aren't compatible with Beta 2 of the 2007 Microsoft Office System.)

Part 1: Fundamentals. If you're new to Open XML, this is the place to start. It's 130 pages, covering the basics of how documents are organized and the underlying concepts such as parts, relationships and content types. A brief overview of each of the main schemas is provided, as well as a summary of the main parts for each type of Open XML document.

Part 2: Open Packaging Conventions. The Open Packaging Convention defines how the ZIP archive that makes up a package (i.e., document) is structured. OPC is used for Open XML documents, and also for XPS documents. This section also contains a bunch of great information about how digital signatures work in Open XML documents.

Part 3: Primer. This section describes, in plain English, how various aspects of Open XML work. If you want to understand how a WordprocessingML document is structured as a collection of stories, runs, and paragraphs, this is where to look.

Part 4: Markup Language Reference. The largest section by far, this is where the individual components of the markup languages are defined: elements, attributes, and types. You can think of this section as low-level reference material for the higher-level concepts covered in the other sections.

Part 5: Markup Compatibility. Developers can extend Open XML by adding their own markup while preserving full compatibility with the Open XML spec. This section covers the concepts behind Open XML's approach to extensibility.

If you've been meaning to learn Open XML, now's the time to dig in and get started. This latest version of the spec is a well-organized learning tool. Take a look, and if you come across anything you don't understand, post a question in the Forums section of OpenXmlDeveloper.org.