Nicomachean ethics.

This collection of books on ethics by Aristotle is still included in many legal ethics required reading lists (which is where I found it), and it's easy to see why. These books clearly articulate the still dominant underlying ethical themes in Western thought.

Unsurprisingly, many sections of this book have not aged well - racism and sexism abound. Many of these examinations also strike as common sense today, although Aristotle makes the case for many ethical norms much more methodically than can be found in popular discourse. That and its foundational place in the ethics canon likely makes it still required reading for people interested in this area

I really liked the Ethics but didn't think this translation was particularly outstanding. I ended up supplementing my reading with many of the other public domain translations available through Google Books. I also liveblogged my study of the book, if you're interested.