APA (7th ed.) Citation
Harley, B. (1990). <The> development of second language proficiency. Cambridge University Press.
Chicago Style (17th ed.) Citation
Harley, Birgit. <The> Development of Second Language Proficiency. Cambridge: Cambridge University Press, 1990.
MLA (9th ed.) Citation
Harley, Birgit. <The> Development of Second Language Proficiency. Cambridge University Press, 1990.
Warning: These citations may not always be 100% accurate.