"On the Shortness of Life" by Seneca, if we can call it a book. The claim that "life isn't short, we just waste most of it" was not by itself that impactful until he started listing examples, among them Caesar Augustus. You can think what you will about him, but nobody can say that he was a lazy man sitting around doing nothing. And yet Seneca shows that Augustus in his "productive" life spent a lot of time complaining how he wished he had more free time, and so he didn't really "live" all the time, just like someone who wastes their days drinking and gambling and whatnot. And the idea that a man who immortalized himself in history for all times "wasted" most of his life was really not something that ever occurred to me. I recommend it to everyone, it's short and written in simple language.