Patton, M. (1990). Qualitative evaluation and research methods (pp. 169-186). Beverly Hills, CA: Sage.
Nielsen, J. (2012). How Many Test Users in a Usability Survey? Alertbox.
OUE Ch 6 (section on recruiting only)
Optional
Brodie, M. (2011) Usability Testing Includes Users as Stakeholders. UX Mag.