I recently completed the above book by Travis Taylor, Bob Boan, R.C. Anding and T. Conely Powell. http://www.amazon.com/Introduction-Planetary-Defense-Extra-Terrestrial-Invasion/dp/book-citations/1581124473
The book makes the argument that life is likely elsewhere, that actual population growth in ecosystems would not follow fermi's paradox, and that given certain assumptions about the density of intelligent life (using Drake's equation) and sub-light travel capabilities, that there is likely a certain frequency of travel, and therefore a non-zero possibility that we could be visited, and that those visiting us would have non-compatible motives. The book spends a fair amount of time on possible motives, and possible defenses. It makes some proposals for how to prepare. Like me, the authors conclude that the highest emphasis should be on propulsion technology. For a good discussion of the problem, see NASA's (now defunct and unfunded) Breakthrough Propulsion Physics:
Overall, I am happy the authors took the risk of introducing a book on the subject, though the book is a bit populist, where I would have preferred something even more academic. The book extensively cites Sci-Fi, which is fine, but would have benefitted from an apology up-front. More time could have been spent elaborating decision trees, and I thought Faster-than-Light (FTL) possibilities were overemphasized. Thankfully, the book did not take a stand on actual visitation, which is characteristic of others that are convinced that such a visit has already occurred ( http://www.exopolitics.org/ )--that will ensure the broadest possible serious audience.
The book would have benefitted by exposure to some Realpolitik theory and a greater expansion of our own history of clashes between exploring advanced civilizations (England, France, Spain, Dutch, U.S., China) and primitive visited civilizations (Incan, Mayan, African, Native North American, Pacific Islanders).
It also only considered contact with a singular entity, and did not discuss the likelyhood that if there is one other spacefaring civilization, there are more than one which likely establishes both motive and possible leverage. It also only discussed warfare only from the perspective of defense, without the positive goal.
The discussion would have benefitted significantly from a reading of Col John Boyd's Discourse on Winning and Losing, where he makes explicit the goal to: Survive, survive on own terms, and improve the capacity for independent action. To read Boyd yourself, go to:
The best way to read up on Boyd is to start with Corum's book Boyd: The Fighter Pilot that Changed the Art of War ( http://www.amazon.com/Boyd-Fighter-Pilot-Who-Changed/dp/0316881465 ), then Hammond's Mind of War, ( http://www.amazon.com/Mind-War-John-American-Security/dp/158834178X/ref=pd_bxgy_b_img_b ) then read Boyd himself (above), and finally, read the Opus,
Osinga's Science, Strategy & Warfare: The Strategic Theory of John Boyd ( available at http://www.routledgestrategicstudies.com/)