I will shameless recommend my own book, Basic and Advanced Focus Groups, from Sage. Each chapter begins with a more basic (introductory) discussion of a key topic, which is followed by an optional presentation of more advanced issues.

Despite the title, here is a general introductory article, found also in researchgate Focus group research: what is it and how can it be used?

• March 2014
• Canadian journal of cardiovascular nursing = Journal canadien en soins infirmiers cardio-vasculaires 24(1):16-22

Are these of any assistance?:

O. Nyumba, T., Wilson, K., Derrick, C. J., & Mukherjee, N. (2018). The use of focus group discussion methodology: Insights from two decades of application in conservation. Methods in Ecology and evolution, 9(1), 20-32.

Article The use of focus group discussion methodology: Insights from...

Caillaud, S., & Kalampalikis, N. (2013). Focus groups and ecological practices: A psychosocial approach. Qualitative Research in Psychology, 10(4), 382-401.

https://halshs.archives-ouvertes.fr/halshs-01206506/file/Caillaud%20%26%20Kalampalikis%202013.pdf

Gibbs A (1997) Focus Groups (University of Surrey)

http://sru.soc.surrey.ac.uk/SRU19.html

Very best wishes,

Mary

I will shameless recommend my own book, Basic and Advanced Focus Groups, from Sage. Each chapter begins with a more basic (introductory) discussion of a key topic, which is followed by an optional presentation of more advanced issues.

There you have it, I was going to recommend Kreuger's and Morgan's work as an answer that is perhaps a cliché given their notoriety, but someone beat me to it :).