The Letter's themes acknowledge the allure of simplicity, controlled emotions, friendship, accepting death, groundless fears, virtuous and courageous actions, rebuking wealth and superfluous materials, practicing poverty, and managing life's setbacks. High level athletes (New England Patriots), world leaders (Bill Clinton), and historical figures (Thomas Jefferson) both utilize(d) and preach(ed) Stoic principles.
I would be doing a disservice advocating only one 'must read' letter, but Letters 13, 18, 27, and 49 will leave you speechless, motionless, and yearning for more. His timeless, pithy, easy to read wisdom consoles all who seek navigating this experiment we call 'life'. You will thank yourself for purchasing this book, setting aside five minutes daily, and exploring the highly therapeutic wisdom. As Seneca would state "Take Care".