Here is a downloadable copy of book “Open XML Explained“.
As you can see, initially this post provided link to e-book for Microsoft’s Open XML (actually there is nothing open there except empty word in it’s name) format developers. But due to righteous criticism the link was removed.
ODF format specifications are available here and here. If it happened so that you have to use M$ Office (in this case I sympathize with you), here is nice utility to get ODF working there (thanks to Wolfgang Lonien for the link).
The OpenDocument format (ODF, ISO/IEC 26300, full name: OASIS Open Document Format for Office Applications) is a file format for electronic office documents, such as spreadsheets, charts, presentations and word processing documents. The standard was developed by the Open Office XML technical committee of the Organization for the Advancement of Structured Information Standards (OASIS) consortium and based on the XML format originally created and implemented by the OpenOffice.org office suite (see OpenOffice.org XML). As well as an OASIS Standard, it is a published ISO and IEC International Standard, ISO/IEC 26300:2006. The OpenDocument standard meets the common definitions of an open standard, meaning the specification is freely available and implementable.