Wong, Jock Onn (2001). To speak or not to speak? The ‘a’ particles of Singlish. National University of Singapore Centre for Advanced Studies Research Papers Series, 37, 33 pp.
A more recent publication building on parts of this one is chapter 7 (pp. 230-259) of:
Wong, Jock O. (2014). The culture of Singapore English. Cambridge: Cambridge University Press. DOI: 10.1017/CBO9781139519519