I picked this up for giggles, and it's a fairly good treatise on how 1) Earth could be defended and 2) How aliens might go about it and more importantly, why? It's technical, and science heavy, but a good read. He demolishes Sagan in two sentences (always good for a laugh) and for $6, it's a great time filler. And, I think, VERY germane to our discussion.
http://www.webscription.net/p-1383-alien-invasion.aspx