Letters to Lucilius

Seconded. For those interested in Seneca's timeless letters but short on time (or if you find existing translations a bit dry), http://stoicletters.blogspot.com/ is a phenomenal resource which concisely but effectively modernizes the language. They're a pleasure to read.

Seneca's Letters to Lucilius helped me come to grips with the modest scale of my own achievements (or lack thereof) and the futility of further ambition at the expense of anything else.