Meditations, originally written by Marcus Aurelius, but translated many times over. More as an introduction to stoicism; choosing the translation that works best for you is ideal. Hays and Hicks translations are most often recommended but if you can peek inside the book to see what language resonates the best with you.

Stoicism helped me build the ability to care about the things in my power to change, and not stress about things that aren't. Very useful in any job or personal situation that includes a lot of ambient stress.