Hellooooo since I was tagged a couple times I might as well give you some resources directly! :^)
Arthurian Legend is a long running literary (and other mediums) tradition with literally endless media. It can be overwhelming but all you can do is get started, there's no pressure to know everything right away or ever!
If you're looking to jump right into reading Medieval literature, I have a Beginner's Guide to Medieval Arthuriana with one hundred links. But to simplify, my opinion for the best places to start, roughly in order of publication, would be:
The Mabinogion translated by Sioned Davies (Welsh)
The Welsh Triads translated by Rachel Bromwich (Welsh)
The Mabinogion and The Welsh Triads are the earliest written stories from the 12th century documented from the Welsh oral tradition. This is where many characters "started." For more of an overview on Welsh Arthuriana, here is The Arthur of the Welsh edited by Rachel Bromwich, the very same who translated Triads linked above.
History of the Kings of Britain (Historia regum Britanniae) by Geoffrey of Monmouth translated by A. Thompson (Latin)
This text is where we first get Arthur's backstory as the son of Igraine, who was the Duchess of Cornwall, and tricked into bed with Uther via Merlin's magic. It's also where the story of Arthur's battle at Mont Saint Michael against a giant comes from, one of Kay and Bedivere's first appearances outside Welsh literature. It also describes Mordred's rise to power and the eventual mutual defeat between Arthur and Mordred. It is the beginning of Arthurian literature's cumulative nature, each story building on the lore of what came before. For more information, here is The Arthur of Medieval Latin Literature edited by Siân Echard.
The Romances of Chrétien de Troyes (French)
There's a "chicken or the egg" situation with three overlapping stories between the Mabinogion and Chrétien's romances, but Chrétien's romances also include Knight of the Cart which popularized the character Lancelot. (Although he first appears in the romance Erec and Enide as well as starring in a German romance Lanzelet around the same time. There are very few absolutes in Arthurian history!)
Merlin and the Grail by Robert de Boron translated by Nigel Bryant (French)
Merlin's orchestral role in destiny? The Sword in the Stone? Christian Grail? Avalon? You can thank Robert de Boron for those motifs. It is among the first complete Arthurian cycles and establishes some of the most famous story elements ever introduced to the legends.
Every text I share from here is based on the foundations laid by the texts mentioned above. You'll begin to see references if not outright repetition as the texts build upon one another and establish what readers today come to expect from any and all Arthurian stories. Honorable mention to Perlesvaus which was also a big influence on the Lancelot-Grail cycle.
The Lancelot-Grail Reader compiled by Norris J. Lacy (French)
The Lancelot-Grail Cycle or the Vulgate Cycle combines all the previous iterations into one long story (divided into 10 books). The head editor Norris J. Lacy has a single volume version with select chapters from the full cycle which is more digestible and enjoyable for someone just starting out. I personally find the Vulgate cycle a very enjoyable read, much like an intricately written fantasy novel.
Parzival by Wolfram von Eschenbach translated by Edwin Zeydel (German)
As Chrétien died before he could finish Perceval: The Story of the Grail, many authors picked up the torch and wrote their own versions; this is one of the most famous and the subject of an Opera as well. It's very long and involved, there are many translations. I prefer it in verse and the attached version is abridged. If you want the full version unabridged, the translation by Jessie Weston is also in verse. For more information, here is The Arthur of the Germans edited by Silvia Ranawake and W. H. Jackson.
La Tavola Ritonda (Italian)
This is the only complete Tristan text in Italian and my personal favorite Tristan text ever. It also covers the full legend including the fall of Camelot after the death of Tristan. It's silly on main and very gruesome. A very fun read. For more information, here is The Arthur of the Italians edited by Gloria Allaire and F. Regina Psaki.
The Wedding of Sir Gawain and Dame Ragnell translated by David Breeden (English)
This is a Middle English poem translated into modern English, a short 30 page read in verse. It's likely inspired by Geoffrey Chaucer's The Wife of Bath's tale and popularized the loathly lady trope in Arthuriana. It's also my favorite. ;^)
Sir Gawain and the Green Knight translated by Burton Raffel (English)
Possibly one of the most famous Arthurian poems of all time. Beautifully written and an iconic story. I link the Burton Raffel version but Simon Armitage translation is very popular, he also has a little BBC documentary he hosts about the poem and the real world locations mentioned in it.
Le Morte d'Arthur by Sir Thomas Malory translated by Dorsey Armstong (English)
The very end of the Medieval texts before the pre-modern era. This is a condensed adaptation of the Post Vulgate, which is itself a condensed adaptation of the Vulgate. It's still going to be very long, and it's personally not a text I enjoy, but the vast majority of subsequent Arthuriana uses this as it's primary source do to wide availability and "completeness" of the story. For more information, here is The Arthur of the English edited by W. R. J. Barron.
HOWEVER! If you're not ready to dive into all these texts yet an want some sort of overview, I have other options or you. One option would be to listen to/watch Dr. Dorsey Armstrong's Great Course's lecture series all about Arthurian legend. Alternatively there's a number of encyclopedic books I can recommend and I have a huge folder of them for you to browse. It also includes some "companion" books to various texts with sprawling narratives that can explain various themes, motifs, historical references, and anything else influential in that text.
Lastly I have some pages on my blog you may like to browse including Character Deep Dives and Research Topics as well as Media Recommendations based on characters or themes (Grail Quest, Welsh Retellings, etc) which extends beyond Medieval literature to include all mediums including novels, comics, films, tv shows, video games, and more.
All of this can be found on my website The Arthurian Preservation Project. I post updates at @arthurianpreservationproject.
I know that's a lot but I hope it gives you some things to start with. Welcome to the community and take care! :^)
