Ruthanna Emrys' post-Lovecraftian novelette The Litany of Earth serves as a starting point for Ada Palmer's polyhistoric thoughts about repurposed fiction, from Geoffrey of Monmouth to Marvels: Discontinuity and Empathy: a non-review of “The Litany of Earth” by Ruthanna Emrys..
Hanover Historical Texts Project is a collection of primary source texts from ancient times to the modern era in English translation. There is a great number of interesting texts, for instance accounts of Zeno, he of the paradoxes, the diary of Lady Sarashina, a lady-in-waiting in Heian era Japan, a letter from Count Stephen of Blois and Chartres, a crusader writing to his wife, Arthur Young's travels in France before and during the Revolution, a report by the American ambassador in St. Petersburg on March 20th, 1917, immediately after the February Revolution, and finally Petrarch's letter about his graphomania. That last one is from what is perhaps my favorite part of the website, a trove of Petrarch's Familiar Letters. But there's much more in the Hanover Historical Texts Projects besides what I've mentioned.