Several books about the endangered language crisis have recently been written for a general audience. Two of the authors, Nick Evans and David Crystal, are interviewed in Language Matters:

Dying Words: Endangered Languages and What They Have to Tell Us. By Nicholas Evans (2010)

Language Death. By David Crystal (2000)

Language in Danger. By Andrew Dalby (2003)

On the Death and Life of Language. By Claude Hagège (2009)

One Thousand Languages: Living, Endangered and Lost. By Peter Austin (2008)

Spoken Here: Travels Among Threatened Languages. By Mark Abley (2003)

The Last Speakers: The Quest to Save the World’s Most Endangered Languages. by K. David Harrison (2010)

Through the Language Glass: Why the World Looks Different in Other Languages. By Guy Deutscher (2010)

Vanishing Voices: the Extinction of the World’s Languages. By Daniel Nettle and Suzanne Romaine (2000)

When Languages Die. By K. David Harrison (2007)
Key guides for individuals and communities working on language revitalization include:

Bringing Our Languages Home: Language Revitalization for Families. Edited by Leanne Hinton (2013)

Can Threatened Languages Be Saved? Reversing Language Shift, Revisited: A 21st Century Perspective. Edited by Joshua Fishman (2001)

How to Keep Your Language Alive: A Commonsense Approach to One-on-One Language Learning. By Leanne Hinton with Matt Vera and Nancy Steele (2002)

Language Endangerment and Language Revitalization. By Tasaku Tsunoda (2006)

Revitalizing Indigenous Languages (full text free online). Edited by Jon Reyhner (1999)

Saving Languages: An Introduction to Language Revitalization. by Lenore A. Grenoble and Lindsay J. Whaley (2006)

Teaching Indigenous Languages (full text free online). Edited by Jon Reyhner (1997)

The Green Book of Language Revitalization in Practice. By Kenneth Hale and Leanne Hinton (2001)
The following books are geared towards linguists interested in the theory and practice of language documentation:

Documenting Endangered Languages: Achievements and Perspectives. Edited by Geoffrey Haig, Nicole Nau, Stefan Schnell, Claudia Wegener (2011)

Essentials of Language Documentation. Berlin, New York: Mouton de Gruyter. (full text free online) Edited by Jost Gippert, Nikolaus P. Himmelmann and Ulrike Mosel (2006)

Field Linguistics: a Beginner’s Guide. By Terry Crowley (2007)

Language Diversity Endangered. Edited by Matthias Brenzinger (2007)

Language Documentation: Practice and Values. Edited by Lenore Grenoble and N. Louanna Furbee (2010)

Linguistic Fieldwork: a Practical Guide. By Claire Bowern (2008)

The Cambridge Handbook of Endangered Languages. Edited by Peter K. Austin and Julia Sallabank (2011)