This book is set in communist China during the Cultural Revolution. This was a time, from 1966 to 1976, when the Chinese Communist Party was trying hard to force all its people to believe in certain basic beliefs that they thought the people were going away from. The Party thought that people were drifting back towards capitalism and they wanted to purge all of that kind of thought and behavior.
This meant that you had better, if you wanted to stay happy and alive, look like you believed everything Chairman Mao Zedong said. That is why the book is so precious. It was a little book of important sayings by Mao. If you did not want to be accused of being a capitalist, you had better have that book and know it well.