Dive into Design Patterns

Dive into Design Patterns is an ebook on design patterns and the principles behind them. Website

"Design patterns help you solve commonly-occurring problems in software design. But you can’t just find a pattern and copy it into your program, the way you can with off-the-shelf functions or libraries. A pattern is not a specific piece of code, but a general concept for solving a particular problem. They are like pre-made blueprints that you can customize to solve a recurring design problem in your code."