Stoicism. Other than old works from Marcus Aurelius, Seneca, Epictetus, you can also find modern stoic practitioners, Massimo Pigliucci and Donald Robertson (there are others, but these are the best at teaching IMO). Robertsons book has additional chapter you can find somewhere online on exercises for meditation on death. But it's better to read it after https://www.amazon.co.uk/Stoicism-Art-Happiness-Teach-Yourse...

Edit. Here it is https://learn.donaldrobertson.name/p/the-stoic-contemplation...

Here some great contemporary introductions to Stoicism:

1. William B. Irvine, "A Guide to the Good Life: The Ancient Art of Stoic Joy", https://www.amazon.com/Guide-Good-Life-Ancient-Stoic/dp/0195...

This is an introduction to Stoic thought as it applies today by a professor in philosophy, very clearly written. Great for first exposure. It (sensibly) skips some of the more arcane stuff, such as Stoic metaphysics (historically relevant, but really obsolete).

2. Donald Robertson, "Stoicism and the Art of Happiness", https://www.amazon.com/Stoicism-Art-Happiness-Teach-Yourself...

This is a touch more academic and historic on one hand, and very practical and text-book-like on the other hand, in that it has self-assessments, key points, exercises for every section. Excellent second book. The author also has a course, blog and FAQ at http://donaldrobertson.name

3. Epictetus' Enchiridion is available on Project Gutenberg, btw. It's very short, and many things are not really relevant today anymore, yet surprisingly many sections still "speak to us".

4. Note also that Tom Wolfe's huge novel "A Man in Full" is suffused with Stoic themes.

I find Stoicism quite wise, and still substantial enough when you subtract all the obsolete superstition (which cannot be said of, for example, Abrahamic religions). Certainly good for tranquility and empathy. Sometimes hard to translate into positive action, though, I find.