Lancelot is the central romance of the Vulgate Cycle, in which the chivalric elements in Arthur's court come to the fore. These chivalric elements contain the seeds of Arthur's destruction and the dissolution of the Round Table, as Lancelot's love for Guinevere undermines his bond to Arthur; the tension between love, prowess and loyalty is the undercurrent of the long romance which describes the exploits which he performs in her service. It also includes many stories which are chivalric adventures largely unrelated to the main theme, and uses the device of interweaving these stories to form a huge stream of narrative. This series of episodic pictures leads ultimately to the birth of Lancelot's son Galahad, who is destined to become the hero of the Grail. Parts three and four of Lancelot begin with the episode of the false Guinevere, in which Guinevere is accused of being an impostor; Lancelot is then abducted and imprisoned by Morgan le Fay, who out of hatred for Arthur intends to reveal their love to the king. When he escapes, Guinevere is abducted by Meleagant, and Lancelot has to rescue her. In the course of these adventures, the Grail appears for the first Lancelot comes to the Burning Tomb, where he learns that his sins will prevent him from succeeding in the Grail Quest; and Gawain reaches the Grail Castle, but fails the test. For afull description of the Vulgate Cycle see the blurb for the complete set.
Norris J. Lacy (born 1940) is an American scholar focusing on French medieval literature. He is the Edwin Erle Sparks Professor Emeritus of French and Medieval Studies at the Pennsylvania State University. He is a leading expert on the Arthurian legend and has written and edited numerous books, papers, and articles on the topic. In 2014 the International Arthurian Society, North American Branch, presented him an award for Lifetime Service to Arthurian Studies.
He received his Ph.D. from Indiana University and has held teaching positions at the University of Kansas, the University of California, Los Angeles, and Washington University in St. Louis. He has served as president of the International Arthurian Society. With Geoffrey Ashe he wrote The Arthurian Handbook, and he edited The Arthurian Encyclopedia and its successor, The New Arthurian Encyclopedia, a standard reference book for Arthurian works. He also oversaw the first complete English translation of the French Vulgate and Post-Vulgate Cycles, released as the five-volume Lancelot-Grail: The Old French Arthurian Vulgate and Post-Vulgate in Translation.
The fourth book of this translation of the Vulgate Cycle covers several episodes of Lancelot's life as an adult and of the Arthurian court, including the death of Galehaut, the imposter Guinevere, and the Knight of the Cart. The latter incident does not differ substantially from its treatment in Chrétien de Troyes' version; the others, I don't know of any sources for them (or, indeed, any other medieval versions of them outside the Vulgate Cycle).
The hinting and teasing about the Grail continues, and towards the end of book IV, Bors comes back into the story. The very end of the book pauses to describe Gawain and his brothers more fully than it has done so far, and introduces Mordred, so I assume he's about to become more important. Or it's just clumsily foreshadowing Mordred's usurpation of Arthur's throne -- which wouldn't surprise me.
As with the other books of this series, it seems to be well translated and the footnotes pick out any areas of confusion, and attribute the statements to whichever manuscript is deemed more likely to be accurate.