The book points out that the real enemy is not meat consumption or production per se but the ecologically disastrous industrialised meat production that comprises over 95% of all the meat eaten in western countries (and an increasing share of the rest of the world too). Such "factory farms" typically involve grain feeding, total confinement systems with liquefied manure, heavy use of hormones and feeding of antibiotics.
Not only are factory farms worse for the planet, they also generally involve much higher levels of animal cruelty.
And so, ecologically speaking, all flesh is not created equal in its impact. The kind of meat and the methods used to raise it are crucial. For instance, there is a huge difference between eating meat from animals culled due to overpopulation (such as kangaroo in Australia, or venison in Scotland) and eating industrially farmed beef or pork. Similarly, there are huge differences between different fish species, some of which are overfished and approaching extinction, while others are carefully managed.
As I've discussed before, I think there is a strong ethical case for serious reductions in the average meat consumption of westerners. This article has reminded me that it is particularly factory farmed meat that is the real culprit (see also here and this book).
How can you tell if the meat you're eating has been industrially farmed? If you have to ask the question, then it almost certainly is. This piece has some good basic suggestions for how to find non-factory meat.