This might be the first self-improvement book ever written. A great Roman emperor, Marcus Aurelius, wrote it for himself nearly 2,000 years ago. Meditations is literally a collection Marcus Aurelius's personal notes and thoughts on how to live life. It's written in simple yet extraordinarily powerful bits, one concept at a time, making it easy to digest.
The book caused an empowering shift in my values and a stronger mental fortitude in me against the effects of things beyond myself and my control. It puts our existence into perspective. He discusses using the thought of death to one's advantage, the irrelevance of the past and future, and the logical handling of emotion in ways a summary of which can do no justice. Essentially, Meditations is dozens of profound concepts, ideas, and strategies, learned over a lifetime of philosophy and leading Rome, rolled into one easily digestible, profoundly useful collection.