•Why it earns a slot
The shortest, most practical entry point into Stoicism ever written, and the direct ancestor of modern cognitive behavioral therapy's core insight: you can't control events, only your judgment of them.
Epictetus was born a slave and later lectured on philosophy; the Enchiridion (Greek for 'handbook') is his student Arrian's digest of his core teaching: almost nothing that happens to you is in your control, but your judgment about what happens always is, and confusing the two is the entire source of human misery.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.