I've recently read two excellent books on endangered languages. Although both take a different approach to this aspect of linguistics both explicitly outline a need for a greener attitude towards linguistics, drawing parallels between the importance of ecological diversity and linguistic diversity, and also highlighting how some aspects of human relationships with the surrounding natural environment may be tied up in the language. Lose some languages and you lose a whole wealth of knowledge about the plants and animals of the surrounding area.
Language Death by David Crystal, published by Cambridge University Press concentrates on the current situation for endangered languages across the globe and considers how some of them may be re-envigorated as living languages.
Language in Danger by Andrew Dalby, published by Allen Lane (Penguin Press) takes a historical approach, for example outlining how the Latin and Greek of the Roman Empire sounded the death knell for the other languages spoken in the areas conquered by the Romans.
For a fascinating book on endangered languages from a case history based perspective, I would strongly recommend Spoken Here by Mark Abley.