By the time Mary Stewart published her standalone novel on the Arthurian legend titled The Prince and the Pilgrim, the first entry in what would come to be known as her Merlin Trilogy had already been around for a quarter of a century. The Prince and the Pilgrim is not one of those examples of a flawlessly executed trilogy being unnecessarily expanded into a full-blown saga by a book that adds to the length of the existing narrative arc without making any serious contribution to its depth.
The Prince and the Pilgrim belongs to a certain type of sub-sub-genre that was still relatively rare at the time the book was published in 1995, but was about to take and is today almost mandatory for series involving purely fictional universes or historical universe populated by familiar characters who are approached in a new. The narrative of The Prince and the Pilgrim takes place within the constructed universe of the books in Stewart’s Merlin Trilogy, but not directly impact the existing trilogy by either continuing the story from a period after where the final book in her trilogy took place or acting a prequel that explains how the characters got to where they are when the reader first encounters them.
Indeed, while it is assumed that The Prince and the Pilgrim takes place during the reign of King Arthur, he is referenced as a character in a way that provides almost no necessary information for understanding any part of the Merlin Trilogy. Likewise, the story of Prince Alexander, the sorceress Morgan Le Fay and the quest of the Holy Grail should not impact any enjoyment or understanding of the earlier tales of Merlin and Camelot.