The go-to one-volume consolidation of the main Arthurian legend is probably Thomas Malory's Morte d'Arthur from the 15th century. Malory streamlines and adds a bit of moralizing to a 13th century series of texts known as the Post-Vulgate Cycle. This was itself an attempt to clean up the first, massive, sprawling endeavour to unite the "Grail romances" and "fall of Camelot" storylines into one. The Lancelot-Grail Reader is a solid translation and abridgement of that set of earlier 13C texts (...the Vulgate Cycle, of course), which might interest you as well.

At the risk of skipping someone's pet text (/u/rimeroyal, want to weigh in?), in general the development of the Arthurian legend can be traced through these texts:

• Geoffrey of Monmouth, History of the Kings of Britain
• Chretien de Troyes, Arthurian Romances, especially "Lancelot" and "Perceval"
• Various versions of Tristan and Isolde; Béroul's is a good one
• (name-check poor Robert de Boron, who probably came up with a lot of the grand narrative)
• the Vulgate cycle, a.k.a. the Lancelot-Grail cycle
• the Post-Vulgate cycle (basically the Vulgate cycle with a lot less Lancelot and a little more Grail)
• Thomas Malory, Morte d'Arthur

The most famous "one-offs" beyond that are probably

• Marie de France, The Lais of Marie de France, which are lovely short fables, some of which are set around the Arthurian court
• Wolfram von Eschenbach, Parzival - the odd non-Anglo/French delegate to the modern Arthurian canon (but note that EVERYONE loved Arthur in the Middle Ages)
• Sir Gawain and the Green Knight

The Once and Future King is a fairly standard entrance into Arthuriana today for people who get exposed to it through reading instead of cultural osmosis, I think. But I'm not that interested in modern (19C+) adaptations beyond Indiana Jones and the Last Crusade, which is the final great Grail romance and I will die on that hill. :)

Chiming in late to the party! I can second the point /u/sunagainstgold makes about Malory's Morte d'Arthur being the best place to look to if you want to see the latest and greatest medieval English group of Arthurian tales that consciously brought together the English and French traditions up to that point into Middle English. Morte d'Arthur was the main source for Tennyson's Idylls of the King (1859) and T.H. White's Once and Future King (1958), which are both where we get most of our modern ideas about the Arthur stories. As far as the most immediate, complete source for Arthur that gave us what we have today, you can't beat it.

The main thing I want to add is that it's also worth looking at the Mabinogion, which was another major influence for Tennyson, if you're interested in tracing the story's "lineage" back a little more. Mabinogion is part of the Welsh tradition, which pre-dates and runs parallel to the Anglo-French tradition, but all the Welsh Arthurian stories are a bit different from the Anglo-French and German ones, thematically. The Arthur we know today is very English, but the Mabinogion is still relevant because it was translated in the 19th century by the Victorian writer Lady Charlotte Guest, whose translation in turn was the version Tennyson turned to when he wrote Idylls of the King.

All of this comes from a very literary genre. There was also an arguably more popular Arthur tradition in the Brut chronicles in the 14th and 15th centuries that English people read as national history, but it's slimmer than the Arthur of romance tradition and not quite as relevant to looking for the source of the Arthur we have today. Worth reading the Middle English Prose Brut if you want a better idea of how medieval people might have thought of the "real" Arthur, but not as useful if you want the biggest chunk of Arthuriana that medieval high literature can give you.

Gah! I can't believe that nobody mentioned Guy Halsall's Worlds of Arthur (2013). Halsall examines what historical evidence there is for an early Arthur character, who presumably would have existed during the Anglo-Saxon period (ca. 500 or so). In typical Halsall fashion, he declares all the evidence we have is mostly crap or misunderstood, and he then attempts to rewrite history using predominantly archaeological evidence.

The legend of course took on a life of its own, and you've got lots of great stuff from /u/sunagainstgold and /u/rimeroyal. I'd personally recommend Malory's Morte, which was my own jumping point (although at times it feels like counting deaths in LOTR), and I enjoyed Sir Gawain a lot, too. But then again, I'm liable to enjoy a good osteological report more than most 13C romances. So if you're willing to risk a foray into the centuries before Geoffrey of Monmouth, Halsall is your guy!