Briefly, the book argues that large-scale locavorism generally fails for economic reasons: it's cheaper and more sustainable to source from around the world, rather than "putting your eggs all in one basket" geographically. (Of course, they are not against sourcing locally when practical, or "gourmet" locavorism, where you pay more to get higher quality foods).
In practical terms, the authors would be for, say, buying meat from New Zealand when you live in North America because NZ has a favorable location for large-scale sheep raising and can thus leverage economies of scale. They are not against buying local meat instead, but feel that because the prices are higher (true as far as I can see), such operations will remain economic niches. They address many of the basic questions about global sourcing; for example, moving large quantities around by boat is generally far more fuel efficient than many such trips (eg, to the regional farm) via automobiles.
I find their arguments sensible but there is something about them that bothers me. Specifically, there seems to be no correlation between healthy food and the economics of food. Margarine is a good "economic answer" for butter (it's cheaper, stores better, etc) but a total FAIL from the Paleo point of view.
Personally, I would describe myself as a gourmet locavore. I pay higher prices for food in exchange for better eats and the hope of future health.
Your thoughts? Generally it helps to have read the book before commenting ;)