I’m trying to write some fiction from the perspective of a character in the 1390s, and I want to read some materials from this time period to get accustomed to the language and attitudes. He’s a world traveler, so writings from any region would work, but I only speak English fluently so other languages won’t be as useful. I don’t need any specific topics either — I have a fairly good understanding of the general historical context of the period. I really just want to get used to how people would write back then to make my work more accurate. TIA!
World traveler in the 1390s?
Then, the closest person you'd expect to find must be Johann Schiltberger (d. around 1440) who was taken as a hostage by the Ottomans at the Battle of Nicopolis in 1396. His work is available as a (unfortunately) dated English translation: Bondage and Travels of Johann Schiltberger, A Native of Bavaria in Europe, Asia, and Africa, 1396-1427, trans. J. Buchan Telfer, London: Hakluyt Society, 1879 (linked to the Project Guthenberg free uploaded ed.).
Alternatively, the memoir of the Castilian diplomat to Samarkand (Capital of Tamerlane) in the first decade of the 15th century (though unfortunately not in the 1390s) is also translated into English and freely available online: Narrative of the Embassy of Ruy. González de Clavijo to the court of Timour, at Samarcand, A.D. 1403–6, trans. Clements Markham. London: Hakluyt Society, 1859 (unfortunately the text is much more dated than Schiltberger's, though).
As for Clavijo, this excerpt of alternative, newer translation by Strange might be rather better: Guy Le Strange, Clavijo. Embassy to Tamerlane 1403-1406 (New York and London: Harper, 1928).
While I fail in finding English (full) translation of Henry Knighton (d. 1396) (famous on his narrative on the aftermath of the Black Death), Older bilingual edition of Chronicle of Adam Usk, Welsh author, is also available online: Chronicon Adae de Usk, A.D. 1377-1421, ed. & trans. Edward M. Thompson. London: Henry Frowde, 1904.
(Adds): If you are interested in the female author, I'd also suggest some works of Christine de Pizan are also translated in English (though not copyright free ed.): Christine de Pizan: A Casebook, ed. Barbara K. Altmann & Deborah L. McGrady. London: Routledge, 2003. NB: she would keep on writing later in the first decades in the 15th century.
When you ask for works "in the English of the time", are you asking for works in Middle English? If so, the best and most widely available 14th century Middle English text is Chaucer's The Canterbury Tales. You can find annotated editions in the original Middle English but also translations into modern English. The Canterbury Tales attempts to portray (and satirize) people from many different walks of life. It will also give you particular insight into the humour of the time. The classic example of a humorous one would be "The Miller's Tale."
Other major 14th century texts in Middle English which wouldn't be too hard to find are Piers Plowman by William Langland and Sir Gawain and the Green Knight by an unknown author. They're very different texts and would give you a good idea of the variety of genres available to writers of the time. Piers Plowman is a satire about the world of the 14th century and takes the form of a sequence of dream visions. Sir Gawain and the Green Knight is an Arthurian romance.
The thing is though, Middle English is pretty hard to read in the original if you haven't studied it. If you are committed to reading in the "English of the time", you definitely want to find a version that glosses the text in the margins.
I would recommend the Book of Margery Kempe! Possibly the first autobiography in English, it records the life of an English woman who traveled widely on pilgrimages and believed she communicated directly with God. The language (Middle English I think?) is difficult but decipherable if you read it in the original, and I think it gives some insights into daily life and really demonstrates the passion with which people approached religion.
It was written in the 1430s, but would cover the 1390s as it was a recounting of her whole life.