If you were intimidated by the large, dense Ecma draft spec for Open XML, things have changed: the latest release, working draft 1.4, is much more accessible.
I've spent the last couple of hours with version 1.4, released last week, and it feels like a completely different document than the previous versions. It's full of useful charts and diagrams, and there are many new sections describing the concepts behind Open XML. It's not quite "Open XML for Dummies" yet, but it's a lot closer to that than the phone-book feel of version 1.3.
Click here to download the spec, which is available in PDF or DOCX format. Note that if you have Beta 2 of the 2007 Microsoft Office System, the DOCX files on the download page won't work because they use the latest revised schemas (which have changed since Beta 2 was locked down).