124 letters of Stoic philosophy addressed to a friend, written by a man who served as Nero's tutor and advisor before being ordered to take his own life.
Seneca's Moral Letters to Lucilius are addressed to a real friend and Roman official, but they read as a sustained, deliberate work of Stoic philosophy in epistolary form -- meditations on death, wealth, friendship, and how to live well under circumstances you don't control. That last theme carried real weight in Seneca's own life: he served as tutor and later advisor to the emperor Nero, accumulated enormous wealth as a result, and was eventually ordered by his former student to commit suicide on suspicion of conspiracy, a death Tacitus later described in harrowing detail. The gap between Seneca's philosophical advocacy of simplicity and his own considerable riches was mocked even in antiquity -- but the letters themselves remain among the most accessible entry points into Roman Stoic thought.