Splitting the books did have an effect on the story. The endings of Swan Knight's Son and Feast of Elfs are both interruptions instead of conclusions, and "commercial breaks" instead of cliffhangers. The questions Jon raised about Gil's imprisonment at the end of Chapter 11 of Swan Knight's Son are immediately answered in Chapter 1 of Feast of Elfs as though both chapters are part of the same episode. This has happened elsewhere in Wright's bibliography, as The Hermetic Millenia and The Judge of Ages are likewise one book published in multiple volumes. In both cases, I recommend that immediately upon the conclusion of the first book, a reader should begin the next.
(Needless to say, spoilers for the next two books are coming. I will minimize them as much as I can)
For it is in Feast of Elfs that we learn that the King Arthur has dominion still on the Earth, and that police offices still hold fealty to the King Under the Mountain. Gil is brought before the Celtic King and Emperor of the Romans and swears fealty to Arthur. But for proper fosterage and knightly training, Gil must return to the Elfish courts. But as he navigates the traps of courtly manners, the Green Knight barges in. As in legend, he tests the assembled knights, seeking one who would cut his head off and receive a similar blow from the Green Knight. Only Gil rises to the challenge, and the Elfs are forced to train him. For the Green Knight has ordered Gil to journey to the Green Chapel in a year's time to honor his oath.
Swan Knight's Sword then describes the perilous journey towards the Green Chapel and the fateful meeting with the Green Knight. Gil endures privations and temptations as he learns the true cost of a knight's oaths.
Even had I not known that Castalia House had split the text of Green Knight's Squire into three books, it is apparent from the text itself. Not only do the books flow seamless into each other, the characters, events, and even the jokes form a loose chiastic. This structure, where pairs of events, themes, or thoughts reflect each other around one central thought - think A-B-C-B-A - is more effective in a single book than across a set of sequels.
Green Knight's Squire is at its heart a coming of age story. At the beginning, Gil is a boy who must learn to become a man. Guided by the example of Arthur and his knights, Gil chooses to become a knight. But a knight, like a man, must be deemed worthy by his seniors, and it is only through hard work and courage that either knight or man can be recognized as such. Mere age does not make a man. So Gil trains and then sets out on a quest that will prove his metal.
The Celtic influence from the Mabinogion manifests in the later books of Green Knight's Squire, and expresses itself even in Gil's coming of age story. Per the Branch of the Mabinogi known as Math fab Mathonwy, a Briton was not a man until he had earned three things: his name, his arms, and his wife. Gil earns each through his travails. He wears the white hilt sword Dyrnwyn, the sword of Rhydderch Hael. Like the Sword in the Stone, Drynwyn can only be wielded by a well-born man, for only then would it burst into flame. Gil earns recognition of his name as the Swan Knight through his courage and courtly manner before the Elfish Kings. And finally, he has the love of Nerea Moth, who, if their puppy love endures, is well on her way towards the chapel.
But Gil is not the only Moth forced to face the challenges of adulthood. Nerea faces her own trial, unique to those who must be left behind. She must unflinchingly bid her knight to accomplish his quest and return to her, knowing full well that he might never return. For if she wavers, so too might Gil's resolve. She must leash her grief and worry, for Gil's morale is more important than her tears. Recognizing that another's well-being is more important than her own is Nerea's own coming of age.
Gil's adventures have proven to be inspirational, a measuring stick against which I can measure my own manhood - and find it lacking. Not all lessons are spoken, and the examples of Gil and Nerea are important in this age where "do what you want it" is the highest goal. This contrasts sharply with the nihilistic Battle Royale inspired murder fests common throughout the Young Adult genre. Only Peter Grant's Maxwell Saga comes close, but while Steve Maxwell's hard work and virtue are rewarded, he is not as striking as Gil in his example to young adults.