Arthurian Folklore: An Introduction.

"He hardly more than touched the sword" - illustrated by Walter J. Enright

King Arthur and His Knights (1903) by Maude L. Radford

King Arthur, like any legendary figure, has so much content written about him that it's hard to know where to begin. These are some resources that can help:

The Camelot Project 
A database for all kinds of information regarding Arthurian canon. Includes archived interviews, artwork, and English translations of some texts.

An Index of Themes and Motifs in Twelfth-Century French Arthurian Poetry (1992) by E. H. Ruck.

A comprehensive listing and description of the small narrative elements in Arthurian poems.

Index des motifs narratifs dans les romans arthuriens français en vers: XIIe-XIIIe siècles (1992) by Anita Guerreau-Jalabert

Same as the above but for Arthurian prose.

Lancelot–Grail: The Old French Arthurian Vulgate and Post-Vulgate in Translation (1992 - 1996) by Norris J. Lacy.
A multivolume translation of one of the most important Arthurian narrative cycle.

From Scythia to Camelot by C. Scott Littleton and Linda A. Malcor (2000)

A book which argues against the supposed Celtic origins of the Arthurian mythos. It claims that the Holy Grail legends derive from the Iranians.

King Arthur in Antiquity (2004) by Graham Anderson.
Same as the above except it proposes an ancient Greco-Roman origin.

The Romance of Arthur: An Anthology of Medieval Texts in Translation, 3rd Edition (2013) by Norris Jay Lacy & James J. Wilhelm.
A collection of Arthurian narratives from Welsh, French, Norse, German and Latin traditions.

The Portuguese Book of Joseph of Arimathaea (2017) by Henry Hare Carter.

A study of an intermediate text that links the death of Jesus Christ with later adventures of Arthur's knights. The actual story can be read in English here.