The Art Of War - Sun Tzu
I served twenty one years in the Air Force and this book was on the recommended reading list for all non-commissioned officers and leaders. I never read it, though I had heard numerous quotes used many times.
I have finally read it. I thought it would be much harder and much more involved. Sun Tzu kept it pretty darned simple...just as a war really would be. Even with it being so concise, there is a lot of information. Though it was written long ago and the battlefield has changed, the lessons taught in this book can be adapted and utilized on a modern battlefield.
I enjoyed every minute of this book and wish I had read it long ago.