THE ART OF WAR by Sun Tzu is something I have read to help give depth to a character in an ongoing WIP. Although the book is an ancient Chinese military treatise, the strategy within can been applied to all areas of life, not just warfare. I'd like to clarify, this is not a review of Sun's Tzu's ability to write, but rather a suggestion to use this book as a way to navigate daily life issues.
You can apply the advice within to everything from professional and personal relationships, political and social change, and any time in your life you face a struggle to achieve your goal(s). However, there are some ideas which are simply irrelevant in modern times, but most of the book is still applicable, centuries later.
The Art of War easily falls into the realm of common sense. Think outside the box. Do not take the bait. Recklessness leads to destruction. Not knowing who you are is a defeat in itself. For some reason, this wisdom is better received from a dead guy than your own friends and family.
If nothing else, Sun Tzu provides some excellent food for thought.