I have mixed feelings about the book. On the one hand, it is a necessary and important intervention into the field of linguistics and an attempt to politicise the study of language as a form of environmentalism for the general interest of society. At the same time however, the book comes short in dealing with the political and economic aspects entailed in cultural and linguistic conservationism, and lacks substance, often times referring to linguistic situations that have either nothing in common with one another (the status of Afrikaans is not equal to the decline of Dyirbal) and which correspond to their own unique and uncomfortable issues. I wasn't entirely convinced that linguists are necessary for the sort of conservationism in question, and the sort of difficulties that Crystal's project will inevitably run into will also clash with the institutionalisation of philanthropy and its mediation through the state. An important shot across the bow, but linguists and language conservation demands more militancy, or at the very least, a reading of Bourdieu's Language and Symbolic Power collection.
EDIT: After some more thoughts and reflection, I feel it is important to also mention that Crystal's suggestions for how to proceed with language revivalism also are problematic beyond a surface analysis. The ethics of linguistic fieldwork are a mess for the same reason that anthropological fieldwork is, but there is a latent idealism that sees the political situation of linguistic communities as an externality, inconvenience, or an unfortunate reality, rather than the constitution of the communities' social spaces as such. This line of surface thinking is harmless until the suggestions for how to proceed with building an educational structure to teach new members of a community the language of their society formally begin to reflect a slightly alienating methodology. Crystal believes, or observes in some sense, that education is universalistic and that teachers need to be educated as one would assume one would teach another human being in how to perform professional labour, without real consideration of the differences and conflicts that can surface when bringing supposedly universalistic pedagogic approaches to communities who face cultural and socioeconomic danger. Whilst Crystal generally does consider that cultural differences will play a role in any form of linguistic conservationism, there is a current which reminds me of humanist Jesuit missions in how it approaches these situations. Western teaching models require the sort of evaluation themselves that they need to be adapted to the context of teaching language, so that they are not subordinated to the same social and political pressures as the Prussian model of schooling enforces (that of instructing children in either some form of classical academic pursuit that idealises the school system as a whole, or one whose explicit aim is merely the future integration into the labour market). I have no doubt that Crystal's intentions are good, but at the same time, the unavoidability of considering the spaces in which language is produced and especially how the sorts of tensions that give rise to the destruction and levelling of languages are recreated in the classroom beyond ignorance, but as structural problems reproducing themselves precisely through the sort of aloof theoreticism that theoretical linguistics is supposed to be confronted with by those working in applied linguistics. In short, one should not rely on this book as a roadmap, but should seek out the work of critical anthropology and pedagogy especially as a complement.