The Prince by Nicolo Machiavelli.  
The Prince is a 16th-century political treatise by Niccolò Machiavelli, serving as a practical guide for rulers on acquiring and maintaining power, often through ruthless or immoral means, which introduced the concept of realpolitik. Published posthumously in 1532, it argues that a ruler must be willing to use deceit, force, and cunning to achieve political ends, prioritizing effectiveness over idealistic ethics. The work is famous for its pragmatic, often controversial, advice and has influenced political thought for centuries.