Officially called Low-Tech Companion 1: Philosophers and Kings (2010) this expands GURPS Low-Tech by taking a look at the "technologies" these two groups need to do their jobs at TL0-4.
"Technology" in this sense is the form of know-how (such as administration, mathematics, writing, religious disciplines, etc) needed to keep things together.
King[]
"King" here refers to the leader even with regards to Stateless Societies. When one comes to States (be they cities or actual countries) actual kings tend to appear.
This chapter goes into how a society would be set up and the duties and responsibilities of the king.
Philosophers[]
Provides details regarding the function of priests, astronomers, mathematicians, and medicine. Details regarding their training is also provided.