An interesting book, although I don't agree with it. Masson seems to claim that the alternatives are factory farms, or the Peaceable Kingdom- and I don't agree. I do not think that factory farms are inevitable; there's an increasing push for more sustainably and humanely-raised meant and animal products (like eggs and milk). Also- and this is not trivial- the Peaceable Kingdom is not actually possible in this reality- enticing though the theory may be.
If creatures went feral and lived in the wild- most would die as a result of being exploited for meat- if not by humans, by wolves or whatever. In addition, most farm animals are technically exotics in the areas in which they are raised, and feral pigs (as an example) are devastating to the ecology of Hawaii. Introducing exotics tends to be A Bad Thing.
And the farm sanctuaries- discussed as an ideal- yet fail for practical reasons to meet the criteria of allowing their critters to live naturally. Either they kill or allow the critters to be killed- or they are over-run. Therefore, the critters are generally not allowed to breed- and according to Masson, this (in theory- he does not apply the theory to the sanctuaries) is a grave crime against the critters' natural instincts.
Basically- most lives rely on the deaths of other things. And the end result of most lives, in the natural (and not Peaceable Kingdom world), is to be eaten by something or other- i.e., to be exploited for their meat.
This book did not convince me that veganism- with all its collateral damage due to growing and harvesting- is actually morally superior to omnivorousness. This was mostly due to Masson's rhetorical tricks, used rather than addressing some of the serious issues raised.
I would recommend Barbara Kingsolver's "Animal, Vegetable, Miracle" as a more balanced approach to what we eat and why.