The Principle Doctrines (Κyriai Doxai in Greek) are found in Lives and Opinions of Eminent Philosophers by Diogenes Laertius.

This collection by Cicero is not about Epicureanism but contains important references.

Lives and Opinions of Eminent Philosophers by Diogenes Laertius (circa 300AD) is a biography of major Greek philosophers, including Book 10 on Epicurus. Contained within are:

• A letter from Epicurus to Herodotus entitled “A Summary of Physical Nature”
• A letter from Epicurus to Pythocles entitled “A Summary of Phenomena of the Sky”
• A letter from Epicurus to Menoeceus entitled How to Live a Happy Life
• The last will of Epicurus, outlining his final thoughts and disposition of his assets (and care of the children)
• The Principle Doctrines

In 1888 a scholar entered the Vatican Apostolic Library — one of the world’s oldest libraries — and opened an ancient 14th century manuscript. (I imagine this like the beginning of some Hollywood mystery movie.)

Therein he found copies of old philosophical texts including the Echiridion of Epictetus, Meditations by Marcus Aurelius — and a previously unknown text entitled “The Sayings of Epicurus.”

The list of sayings was collected or copied by an unknown author but is generally attributed to Epicurus of Samos, the founder of ancient Epicureanism.

On the Nature of Things by Lucretius (circa 50BCE) is an epic poem exploring Epicurean physics.

On the Ends of Good and Evil is another major work by Roman author Cicero. Books 1 and 2 present a dialogue regarding Epicureanism.

On the Nature of the Gods by Cicero is written as a dialogue between Cicero and representatives from Epicurean, Stoic and Academic Sceptic schools. Book 1 contains the discourse of Velleius — a Senator and Epicurean — which consists of three parts: a general attack on Platonist and Stoic cosmology; a historical review of the earlier philosophers; and an exposition of Epicurean theology.

Diogenes of Oenoanda was an epicurean in modern day Turkey (circa 200AD). A wealthy man, and concerned epicurean, he commissioned an inscription that explains the teachings of Epicurus — on physics, epistemology, and ethics. It was originally about 25,000 words long and filled 260 square meters of wall space. Less than a third of it has been recovered.

The Villa of the Papyri is an ancient Roman villa, named after its unique library of papyri (or scrolls), discovered in 1750. It contained over 1,800 papyrus scrolls, now carbonised by the heat of the eruption, the “Herculaneum papyri.”

Epistulae Morales ad Lucilium better known as Letters from a Stoic, is a collection of letters by Seneca the Younger. Seneca is one of the top Stoic writers, but this book contains ~30 quotations of Epicurus.