Kumar's lecture notes, Lecture 2 and Lecture 3
on MSO on finite words and its equivalence with regular languages (In Chapter 3, omit Section 1
on Stratification of FO formulas).
Notes from CS 474 Fall 2021 by Mahesh Viswanathan and Madhusudan Parthasarathy. See Appendix A for resources on Turing Machines and Complexity Classes.