John Gerring (2007): "It really is an intensive study of only one unit or a small amount of models (the cases), which seeks to acquire a deep knowledge of that device or models." You'll be able to read the preface and Chapter 1 of Yin's e-book here. See the https://garrettvrpuy.tinyblogging.com/the-smart-trick-of-case-study-analysis-that-no-one-is-discussing-78496472