From the textbook, Operating Systems: Three Easy Pieces, there are readings that address the concepts we discuss in class (and sometimes implement in projects). Reading these can fill in details and provide another perspective to understand the design and implementation questions.
Introduction / Overview: Chapter 2, through (and including) Section 2.6.
Processes: Chapters 4 through 7. Chapters 8 and 9 go beyond what we have covered, so explore them, too, just for the fun of it!