A good background for grasping the material in this Lesson
can be obtained by reading Chapter 5 (Unemployment) and Chapter 12
(Aggregate Supply) of Greg Mankiw's intermediate text book
Macroeconomics, which is published by Worth Publishers.
You should also download and read the first essay in John E. Floyd,
Expositional Essays in Macroeconomics.
Also, those who have trouble understanding the tiny bit of calculus used
in this Lesson should work through the relevant material in my
Mathematics Review for Economists.