Labov, W. (2001). Principles of Linguistic Changes: Social Factors. Malden, MA: Blackwell Publishers.
Lakoff, Robin T. (2000). The Language War. Berkely, CA: University of California Press. ISBN 0-520-21666-0
Meyerhoff, Miriam. (2006). Introducing Sociolinguistics. New York: Routledge. ISBN 0-415-39948-3
Milroy, Lesley and Gordon. Matthew. (2003) Sociolinguistics: Method and Interpretation London: Blackwell Publishing. ISBN 0-631-22225-1. (More advanced, but has lots of good examples and describes research methodologies to use.)
Paulston, Christina Bratt and G. Richard Tucker, editors. 1997. The early days of sociolinguistics: memories and reflections.(Publications in Sociolinguistics, 2.) Dallas: Summer Institute of Linguistics.
Trudgill, Peter. (2000). Sociolinguistics: An Introduction to Language and Society(4th Ed.). London: Penguin Books. ISBN 0-14-028921-6 This book is a very readable, if Anglo-centric, introduction for the non-linguist.
Watts, Richard J. (2003). Politeness. Cambridge: Cambridge University Press.ISBN 978-0-521-79406-0. A sociolinguistics book specializing in the research in politeness. It's a little tough at times, but very helpful and informational.