The letters
Gaius Plinius Caecilius Secundus published nine books of letters, carefully selected and polished for publication, covering law, politics, literature, estate management, patronage and friendship. A tenth book preserves his official correspondence with Trajan as governor of Bithynia-Pontus, with the emperor's replies — a unique record of Roman provincial administration in operation.
Vesuvius
Two letters to Tacitus, written some twenty-five years after the event, describe the eruption of 79 CE. The first recounts the death of his uncle, Pliny the Elder, who took ships across the bay both to observe the phenomenon and to evacuate people, and died on the shore. The second describes the writer's own escape from Misenum with his mother.
They are the only eyewitness account of the eruption, and they are the reason the eruption column type is named Plinian. They are also retrospective literary compositions written at Tacitus's request for a history, which is worth remembering when they are quoted as reportage.
The date they give, in the manuscripts as transmitted, is 24 August. A substantial body of physical evidence points to autumn, and many specialists now treat the transmitted date as a manuscript corruption.
Why he appears in the architecture layer
For his villa letters. Two long descriptions of his own houses, room by room, with orientation, views, heating and the reasons for each arrangement, are the fullest account of how a Roman of means thought about domestic space — and a useful corrective to reading villa plans without knowing what they were for.