Vulgate Cycle

The Vulgate Cycle, also known as Lancelot-Grail, the Prose Lancelot, and the Pseudo-Map Cycle, is in five parts:
The History of the Holy Grail
The History of Merlin
Lancelot (proper)
The Quest for the Holy Grail
The Death of Arthur

It has only recently become available in English, in five large volumes containing both the Vulgate and a significantly rewritten version called the Post-Vulgate.