The following article contains an answer for the question: What is the name of the work of Geoffrey's that it's immense popularity cannot be denied? , can you please find it? 

How much of this narrative was Geoffrey's own invention is open to debate. He seems to have made use of the list of Arthur's twelve battles against the Saxons found in the 9th-century Historia Brittonum, along with the battle of Camlann from the Annales Cambriae and the idea that Arthur was still alive. Arthur's status as the king of all Britain seems to be borrowed from pre-Galfridian tradition, being found in Culhwch and Olwen, the Welsh Triads, and the saints' lives. Finally, Geoffrey borrowed many of the names for Arthur's possessions, close family, and companions from the pre-Galfridian Welsh tradition, including Kaius (Cei), Beduerus (Bedwyr), Guenhuuara (Gwenhwyfar), Uther (Uthyr) and perhaps also Caliburnus (Caledfwlch), the latter becoming Excalibur in subsequent Arthurian tales. However, while names, key events, and titles may have been borrowed, Brynley Roberts has argued that "the Arthurian section is Geoffrey's literary creation and it owes nothing to prior narrative." Geoffrey makes the Welsh Medraut into the villainous Modredus, but there is no trace of such a negative character for this figure in Welsh sources until the 16th century. There have been relatively few modern attempts to challenge the notion that the Historia Regum Britanniae is primarily Geoffrey's own work, with scholarly opinion often echoing William of Newburgh's late-12th-century comment that Geoffrey "made up" his narrative, perhaps through an "inordinate love of lying". Geoffrey Ashe is one dissenter from this view, believing that Geoffrey's narrative is partially derived from a lost source telling of the deeds of a 5th-century British king named Riotamus, this figure being the original Arthur, although historians and Celticists have been reluctant to follow Ashe in his conclusions.Whatever his sources may have been, the immense popularity of Geoffrey's Historia Regum Britanniae cannot be denied. Well over 200 manuscript copies of Geoffrey's Latin work are known to have survived, as well as translations into other languages....

Historia Regum Britanniae