Meditations by Marcus Aurelius.

Stoicism is by far the best philosophy I've ever encountered. Some people call it "the best operating system for the mind", and I very much agree with that statement.

It changed my life more than any other corpus of ideas. I can't overstate how much better I feel now that my brain is running on a 'Stoic OS', especially on an emotional level --which was the hardest to deal with as I'm rather hyper-sensitive; now my emotions have truly become an almost entirely positive force in my experience of life, regardless of their nature, good or bad, of said emotions; in fact I no longer even qualify emotions on this scale; and the same goes true for an overwhelming majority of my thinking.

This book is the personal journal of one of the greatest roman emperors, leader of the (western) world at the time. A rare enough occurrence in the history of leaders, he was deemed 'worthy of his position' on a human and philosophical level by most people who knew him.

A couple remarks: "philosophy" as seen by ancient authors and thinkers is not a strictly intellectual or abstract endeavor, not a scholarly matter, at least not at its core. Philosophy is the closest equivalent they had to what we'd call "self-development" today. It's very down to earth, 'life recipes' of sorts, simply to educate and help people deal with this elusive brain of ours. Seneca's and Epictetus writings are also excellent food for thought, food for one's mind. Imho, philosophy, litterally the "love of wisdom", is something we should deeply reappropriate, both as individuals and whole societies.

Relatingly, Stoicism used to be taught from childhood throughout most of human history in the western world (and it could be argued that Asia has its own equivalent philosophies). For some reason, we ceased teaching philosophy to children around the turn of of the 20th century, which leaves most people with a lack of means to deal with their emotional circumstances. I'm one of those who consider this to be a dire pity, especially in our day and age. I think it sorely shows in public discourse and interpersonal relationships, and the end result is too much suffering that is entirely preventable.

Meditations is easy enough to read, but if you want something more modern, I found The Obstacle is the Way by Ryan Holiday to be a very good introduction to Stoicism.

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As A Man Thinketh by James Allen. It's as short as it is good for the mind, the building/making of one's persona. Well worth a read at least once in your life, there are many 20th and 21st century self-development books (e.g. How to Win Friends and Influence People) that I believe drew some of their teachings from this 1903 classic.

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The Power of Now by Eckhart Tolle. Regardless of where you find the information contained in this book (there are many works on the topic, both modern and throughout history), this book helps understanding that living in the present moment is critically important, and a key to happiness. Notwithstanding the 'wu-wu' aspects of Tolle's particular take, it just works. If you find yourself constantly dwelling on the past, or being a 'nostalgic of the future' (as I both used to do), knowing the value of living 'in the now' may be the difference between chronic depression and a fulfilling experience of life. It certainly is for me.