Research on the role of motivation in second language teaching and learning dates to early work by Canadian social psychologists Howard Gardner and Wallace Lambert:
Gardner, H., & Lambert, W.E. (1959). Motivational Variables in Second-Language Acquisition. Canadian Journal of Psychology, 13 (4), 266-272 PDF
An accessible overview of Gardner's more recent work is provided by Norris-Holt (2001) in Motivation as a contributing factor in second language acquisition.
Further reading:
Crookes, G., & Schmidt R.W. (1991). Motivation : Reopening the research agenda. Language Learning, 41(4), 469-512.
Dornyei, Z. (2005). The psychology of the language learner. Mahwah, NJ: Lawrence Erlbaum Associates.
Gardner, Tremblay, and Masgoret (1997). Towards a full Model of Second Language Learning: An Empirical Investigation. The Modern Language Journal, 344-362.
Masgoret and Gardner (2003). Attitudes Motivation, and Second Language Learning: A Meta-Analysis of Studies Conducted by Gardner and Associates, Language Learning. 167-21