I'd like to mention two books because I can't decide which is greatest (they're very different):
- The Checklist Manifesto by Atul Gawande (often quoted here, and rightly so; it's short and really really great)
- How to Live, or A life of Montaigne by Sarah Bakewell (a fantastic take on Montaigne's essays by a contemporary scholar with a refreshing take on everything).