All Arthurian literature is based, directly or indirectly, on legends that were first recorded in the Middle Ages.
In addition to reading the source materials (some of which you will find in the tabs in this section) there are several interesting scholarly books on the subject. One such book is Arthurian Literature in the Middle Ages, A Collaborative History, edited by Roger Sherman Loomis. The book includes thirty essays by Arthurian scholars, and examines the Welsh poems (see tab for more information), the early poems about Tristan & Isolde, and the origins of the Grail legend.
For anyone interested in a scholarly examination of the medieval sources of the Arthurian literature, this might be a nice read.
Another scholarly book on the Arthurian legend is The Cambridge Companion to the Arthurian Legend, edited by Elizabeth Archibald and Ed Putter. The book examines the early written sources as well as more recent ones, and traces the legend’s prominent themes. This book covers more material than the aforementioned one, but the prior book goes into more depth on medieval sources.
Another book that gives scholarly insight into the medieval souce material is The Oxford Guide to Arthurian Literature and Legend, by Alan Lupack. This book is especially interesting because it separately examines the characters, places, and symbols.