Software's Primary Technical Imperative is managing complexity. This is greatly aided by a design focus on simplicity.
Simplicity is achieved in two general ways: minimizing the amount of essential complexity that anyone's brain has to deal with at any one time, and keeping accidental complexity from proliferating needlessly.
Design is heuristic. Dogmatic adherence to any single methodology hurts creativity and hurts your programs.
Good design is iterative; the more design possibilities you try, the better your final design will be.
Information hiding is a particularly valuable concept. Asking "What should I hide?" settles many difficult design issues.
Lots of useful, interesting information on design is available outside this book. The perspectives presented here are just the tip of the iceberg.