The Letters (Latin: Epistulae) of Pliny the Younger (61-c.113 CE) stand alongside those of Cicero as one of the most intimate records left to us by any individual from the ancient world, although the fact that Pliny published them during his lifetime should give us pause about how candid a picture they really present.
Pliny was a politically active member of the equestrian order, who rose to become governor of Bithynia under Trajan. His correspondence with the Emperor famously illustrates the centralisation of Roman administration with advice sought and given on matters great and small. This includes a discussion of how to deal with the rising sect of Christianity, an important piece of evidence on the early history of the Church.
The Letters also shed light on Pliny's role in the leading literary circles in the time. A number of them show Pliny in the role of patron to Suetonius, who may have served on his staff in Bithynia. It was in a letter to another historian, Tacitus, that Pliny recounted the death of his uncle Pliny the Elder, the author of the Natural History, while attempting to investigate the eruption of Vesuvius.
Free online texts
Gutenberg: Letters of Pliny.
Glossa.dk: C. Plinii Caecilii Secundi epistulae. Latin text, html format.
Internet Classics Archive/Internet Archive: Pliny's Epistles In Ten Books, translated 1723. HTML format.
Latin Library: Epistularum Libri Decem. Latin text, HTML format.
Loebulus. L055 - Pliny the Younger -- Letters I: Books 1-7. Also available at the Internet Archive.
Loebulus. L059 - Pliny the Younger -- Letters II: Books 7-10. Also available at the Internet Archive.
Perseus: Letters. Latin text, html and xml formats.
Pomona.edu (Trevor Chinn): Letters Books 1-5 (complete), Letters Books 6-9 (excerpts) and Book 10 (complete). English text, HTML format.
BBC Radio 4 In Our Time: Pliny the Younger. Melvyn Bragg with Catharine Edwards, Roy Gibson and Alice König .
Librivox: Letters of Pliny - public domain audiobook.
Livius: Pliny the Younger.
Wikipedia: Epistulae (Pliny).
The Great Conversation: Further reading at Tom's Learning Notes
Pliny the Elder: Natural History.
Tacitus - Historian and friend of Pliny the Younger.
Suetonius - Another friend, and perhaps client, of Pliny.
Latin Resources: Online materials for learning Latin.