Datacenter Resource Site - General Design
Sean Donelan's Classification of Data Centers
Martin Hannigan's Fugawi Networks Qualifications to Bid on Projects
Jeff Albro's Datacenter Design Outline
Nanog 14 Datacenter BOF Notes
Sun Datacenter Plannning Guide
Microsoft Data Center 1.5 Solution Documentation
This is a collection of Windows binaries, which I can't read on my Linux laptop, so I can't speak to its value.
US Department of Energy Standards
some of which are relevant, many of which are not
BISCI
Handbooks for Sale, Links, Presentations, Conferences and Training; Links to many standards organzations
The Uptime Institute
some useful whitepapers, seminars