What is multilevel modeling and why should I use it?
Robson, K., & Pevalin, D. (2016). What is multilevel modeling and why should I use it? Multilevel modeling in plain language (pp. 1-20). SAGE Publications Ltd.
This book covers multilevel technique without emphasizing all the statistical equations and strange notations that are typically associated with multilevel modeling. The book demystifies these types of approaches for people who are intimidated by technical language and mathematical symbols.
Read Chapter 1: What Is Multilevel Modeling and Why Should I Use It?