McQuarrie E. F. Customer Visits: Building a Better Market Focus. Newbury Park, Sage, 1993.
[Very well written; the topic is somewhat specialized, but many of the points apply to qualitative research in general.]
If you want need a good marketing research reference text you might invest in Churchill's Marketing Research.
[It has everything (almost). Also, Churchill is the required textbook for 15.822.]
Churchill, Gilbert A. Marketing Research, 6th Edition, Chicago: The Dryden Press, 1996.
[Plus: Comprehensive and precise. Minus: Dry, expensive.]
Lehmann, Donald R. Market Research and Analysis. 3rd Edition, Homewood, IL: Irwin, 1989.
[Plus: Well written, sensible. Minus: Virtually no coverage of qualitative methods.]