After Marcus Aurelius, this is one of my favorite books. While Marcus wrote mainly for himself, Seneca had no trouble advising and aiding others. In fact, that was his job—he was Nero’s tutor, tasked with reducing the terrible impulses of a terrible man. His advice on grief, on wealth, on power, on religion, and on life are always there when you need them. Seneca’s letters are the best place to start, but the essays in On the Shortness of Life (Amazon) are excellent as well. You can draw a pretty straight line from Seneca to the essays of Montaigne (also read: How To Live (Amazon), a biography of Montaigne) to the modern day writings of Nassim Nicholas Taleb (read: The Black Swan (Amazon), Fooled By Randomness (Amazon) and The Bed of Procrustes (Amazon)).