Here is an ideal textbook for an introductory course on variation, as well as a useful resource for scholars with some background in linguistics who are interested in the study of language variation and its relation to the wider field of linguistics.
James A. Walker is Associate Professor of Linguistics in the Department of Languages, Literatures, and Linguistics at York University in Toronto. His area of research is the quantitative analysis of sociolinguistic variation, including interests in language contact, bilingualism, ethnicity, pidgins and creoles, as well as the relationship among phonology, morphology, and syntax.