"Design Patterns: Elements of Reusable Object-Oriented Software" by the Gang of Four. It has sold over 500k copies and has been translated to numerous languages.
There are only five patterns that matter in real world software development.
Observable
Singleton
Composition
MVC
Inversion of Control
And the last one isn't even covered by GOF. The book is an out of date relic of 90s programming. Let it die already.
If anything, that suggests to me that it is not. "Philosophy for Dummies" far outsells Plato.