The Gang of Four's seminal catalog of 23 patterns to solve commonly occurring design problems

Patterns allow designers to create more flexible, elegant, and ultimately reusable designs without having to rediscover the design solutions themselves. Highly influential, Design Patterns is a modern classic that introduces what patterns are and how they can help you design object-oriented software and provides a catalog of simple solutions for those already programming in at last one object-oriented programming language.

Each pattern:

From the preface: "Once you the design patterns and have had an 'Aha!' (and not just a 'Huh?') experience with them, you won't ever think about object-oriented design in the same way. You'll have insights that can make your own designs more flexible, modular, reusable, and understandable - which is why you're interested in object-oriented technology in the first place, right?"