If you haven't heard, Open XML has created a lot of buzz on it way to becoming a standard.
Back at ODC, I was able to bring back a couple of copies of Open XML Explained, a book talking about the specifications (written by Wouter van Vugt). You can hit the link above to download a copy, or ping me if you want a paper copy (very limited supply).
As well, check out Open XML SDK as well as the recent announcements about the roadmap (v1 release in May 2008, v2 CTP in the summer, v2 release with Office "14").
Can you create Office documents without Office (even on the server or client)? As an example, check out Wouter's Package Explorer.