There is a wealth of literature into the key principles of variation theory and the choice of examples in general. Some of my favourites are:

• What is made possible to learn when using the variation theory of learning in teaching mathematics? by Angelika Kullberg, Ulla Runesson Kempe and Ference Marton
• Exemplification in Mathematics Education by Liz, Bills, Tommy Dreyfus, John Mason, Pessia Tsamir, Anne Watson, Orit Zaslavsky
• Action Research on the Application of Variation Theory in Mathematics Teacher Education by Leicha A. Bragg
• Shedding Light On and With Example Spaces by Paul Goldenberg & John Mason
• Getting Students to Create Boundary Examples by Anne Watson and John Mason
• My podcast interview with Anne Watson and John Mason
• The ATM book that Anne edited: Variation in mathematics: A collection of writings from ATM Mathematics Teaching, 

And I wouldn’t be much of a salesman if I did not take this opportunity to recommend my own book, Reflect, Expect, Check Explain, or my online courses which are both jam-packed full of research.