Frank Rice has a great article entitled "How To: Manipulate Office Open XML Formats Documents" on MSDN.
It goes through a series of instructions for combining sample document parts (provided) to manually create rich documents, then a series of instructions for editing documents manually. Since these are manual exercises, non-programmers or programmers working on any language or platform can learn about Open XML architecture from them.
Next comes a section on writing code to manipulate Open XML documents, using the System.IO.Packaging API in the .NET 3.0 Framework. The article takes you step by step through creating a fast simple text-search utility as a Winform application.
Nice job, Frank! By the way, when you localize this to Spanish, will the graphic be a snake instead of a bald eagle? 🙂