I'll second raid's recommendation for the ASHRAE materials. Also, you may want to check out
The Green Grid, the
Rocky Mountain Institute and
Berkeley Labs. Also, James Hamilton from Amazon Web Services has lots to say on this topic, so you might look around on his blog:
http://perspectives.mvdirona.com/
Yahoo and Google are making remarkable strides in data center efficiency, so you might want to Google their efforts. Of course, the magnitude of their efforts may not scale down to levels usable by us mere mortals. This raises the question of long-term economic survivability for small, private data centers.
The trouble with traditional books is that they take a long time to publish and these changes are happening very quickly.
Ken