The basic premise is that the core models on which we've long built macro economic theories are far too simplistic and therefore wrong. Markets of all types are influenced dramatically by "animal spirits" (aka human psychology) and we need to find ways to successfully incorporate this new understanding. The authors suggest that theirs is a unique insight and as a result we can finally understand why our macro econ models are so broken: "Our theory of animal spirits provides an answer to a conundrum: Why did most of us utterly fail to foresee the current economic crisis?" I found this statement a bit too bold, shouldn't behavioral economics pioneers like Daniel Kahneman and Vernon Smith share in the credit? Also beyond a light weight discussion of their theory and it's grand implications there isn't much depth to back up their claims (though I'm not saying the ideas are wrong). Further research is left as an exercise to the reader or the current crop of grad students...
If you haven't been following the latest economic thinking since undergrad (and you went to undergrad 10+ years ago) you might find this book illuminating, otherwise I'd probably skip it.