Case studies can be another form of grey literature and provide another level of research for you to build upon. According to Crowe, et al. (2011), authors of The Case Study Approach, "A case study is a research approach that is used to generate an in-depth, multi-faceted understanding of a complex issue in its real-life context." They go on to say that it can be also be defined as, "the need to explore an event or phenomenon in depth and in its natural context. It is for this reason sometimes referred to as a "naturalistic" design; this is in contrast to an "experimental" design (such as a randomised controlled trial) in which the investigator seeks to exert control over and manipulate the variable(s) of interest" (Crowe, et al., 2011).
To learn more about Case Studies read about it in the chapter called "Case Study," by Brent Flyvbjerg in the 2011 book, The SAGE Handbook of Qualitative Research here.
Crowe, S., Cresswell, K., Robertson, A., Huby, G., Avery, A. & Sheikh, A. (2011). The case study approach. BMC Medical Research Methodology. (11) 100. https://doi.org/10.1186/1471-2288-11-100