To write a book that claims to give an account of a field and its terminology is inevitably to expose oneself to criticism. Why does Garrard write about this and not that? Does this particular argument really deserve the airtime it gets? I imagine we could go on like this forever, because the boundaries of ecocriticism are often contested, and some people who could be said to do ecocriticism don't call themselves ecocritics. There were some things about this book that I wish were different based on my own particular viewpoints.
But to evaluate this book fairly we have to evaluate it sympathetically, and to do that it is worth reminding ourselves of what this book is. Garrard's book should be seen as a kind of gateway to ecocriticism, and it is a very good one. Garrard provides thorough glosses of the concepts and problems that drive ecocriticism, sketches out the various positions that might fall under the ecocriticism label, and gives a detailed account of the thought that forms ecocriticism's theoretical underpinnings.
I have the second edition, which covers the major developments in the field over the course of the 2000s, and so Garrard's book is more useful for a scholar finding his or her way into the field than, say, The Ecocriticism Reader: Landmarks in Literary Ecology which, given that its role was to get a scholarly conversation started, is now somewhat dated.
Because this book is mostly about giving a gloss to a body of scholarship, it is not eminently citable if one is actually doing research. I would also only make it optional in a classroom, as a seminar discussion would be better spent on an actual literary or theoretical text than a gloss. The literary analyses model point to ways of doing ecocritical inquiry into different genres, but they can also be hard to evaluate and may be of little interest if you don't know the texts. My biggest criticism is that some of the analyses remain underdeveloped in this new edition, and I wish that there were more of a dissection of some ecocritical commonplaces that we really ought to have dispensed with by now..
What is most valuable about this book is that it points the way to a genuinely critical mode of ecocriticism, as opposed to art appreciation or commentary, or worse, what has been called "the praise song school of ecocriticism." Appreciation and commentary are a necessary phase of starting a conversation, but doing that kind of scholarship forever is rather pointless. If you are a beginner broaching the field of ecocriticism, you can save yourself some time by starting with this book. And if you are more familiar with the territory, this book is still worth having in reach, if only because it is useful in pointing out possible directions through the kinds of conceptual problems one bumps into when undertaking ecocritical scholarship, however one wishes to define that term.
This book can't be everything to everybody interested in ecocriticism, but that's probably a good thing for ecocriticism. The fact that ecocriticism can't be reduced to a two hundred page volume is surely indicative that there is enough heterogeneity to sustain it as a viable mode of critical inquiry.