•Why it earns a slot
The book that invented modern social contract theory: the idea that legitimate government exists only because people agreed to trade some freedom for protection, still the starting point for every debate about the proper limits of state power.
Hobbes wrote Leviathan while England was tearing itself apart in civil war, and it shows: he argues that without a single overwhelming authority, human life collapses into a war of everyone against everyone. His solution is a social contract so total that every person surrenders their right to self-government to one sovereign, in exchange for basic safety.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.