Hi,
It depends on your area and what analysis you plan on doing, however, for my health services-related PhD I've found the following very useful:
Ritchie, J. & Lewis,J. (2004). Qualitative Research Practice (this is a really nice clear book, good for theory but quite a practice focus--I use it a lot)
Bryman, A. (2004). Social Research Methods
Hansen, E.C. (2006). Successful Qualitative Health Research.
Bowling,A. (2002). Research Methods in Health: Investigating health and health services.
In my early days I also found the "Qualitative research methods in health technology assessment: a review of the literature" monograph really interesting for background and theory on qualitative research, see http://www.hta.nhsweb.nhs.uk/execsumm/summ216.htm