Dive into SpreadsheetML Part 1   Part 2
by Erika Ehrli (Microsoft Corporation ^TM)

Simple SpreadsheetML file Part 1  Part 2   Part 3
by Brian Jones (Microsoft Corporation ^TM)

Open XML Explained e-book
(by Wouter Van Vugt)

Final Drafts for Ecma Office Open XML File Formats Standard

Ecma whitepaper on Open XML

Ecma Office Open XML Formats frequently asked questions

Links

OpenXML Developer

Microsoft Office Excel 2007