The Letters to Lucilius – an introduction to Stoicism – by Seneca the Younger, trans. Richard Mott Gummere
From Project Gutenberg:
Selections from the Discourses of Epictetus, with the Encheiridion (“The Handbook”), by Arrian, trans. George Long
Meditations, by the Emperor Marcus Aurelius, trans. unidentified
The Memorable Thoughts of Socrates, by Xenophon, trans. Edw. Byssche.