Marcus Aurelius' Meditations is absolutely a must read for anyone interested in philosophy or the history of philosophy, as well as anybody just looking for some self help knowledge.
It's one of the few still surviving works from the stoics. It preaches a general message of tranquility being superior to happiness or pleasure. The primary method of achieving tranquility is to disregard your own opinion and align your behavior with the principles of nature/God, which is a message that only becomes more truthful as modern society declines into narcissistic and atheistic ways. He attributes value to what he calls "social" behaviors, by which I think he means participatory behaviors, or engaging with the natural order positively. In other words it is important to participate and contribute to the greater order around you in order to complete your position in the world and achieve tranquility.
It has a ton of little chunks of wisdom, similar to the bible, easily grasped sort of socratic arguments like "Why do we value ourselves so much yet favor other people's approval over our own tranquility?" and "Are you not content simply with doing what is according to your nature, why do you seek to be paid for it?". The book is pretty much just brief but meaningful observations like these, so you aren't expected to comprehend some complex system of philosophy, though you can if you want to. Due to this it is a very accessible book at several levels, you can read it and confidently gain some useful knowledge no matter how deep you are in philosophy as a whole.
Overall, it's very good and offers a very interesting traditional approach to self help that is lost in modern pop psych literature. It touches on tons of topics so theres something in it for everybody, would recommend.