It's not a lot, but there are a bunch of pictures on google.com/about/datacenters, including a 'street view' inside our DC in North Carolina.

There are also a bunch of papers on the network architecture, https://research.google.com/pubs/Networking.html .

But yes, a lot is closed for the obvious reason that this is really core to the business Google is running.

(I'm a datacenter engineer at Google)