Almost forty years old, he then specialized in children's fantasy, the genre of his best-known works. His wartime tenure in Wales introduced him to castles and scenery that would inspire settings for many of his books. Alexander was particularly fascinated with Welsh mythology, especially the "Mabinogion". The plot for "The Book of Three" is based on a fragment from the "Myvyrian Archaiology". Alexander signed a book deal with Henry Holt and Company for a trilogy called "The Sons of Llyr". Alexander resisted simplifying the Welsh names, stating that they gave the book a certain mood and strangeness. After the release of the first novel, "The Book of Three" (1964), the series became known as "The Chronicles of Prydain". The second book of the series, "The Black Cauldron", followed in 1965. After beginning the third book "The Castle of Llyr" (1966), Alexander decided his story needed to be told in four books, not three, and he planned his fourth and final novel "The High King of Prydain". During this time he also worked at the "Delaware Valley Announcer" as an associate editor. After having a near-death experience, Alexander hastily finished "The High King", concerned he would be unable to finish his saga. However, his editor Ann Durell suggested that he write a fourth book in-between "The Castle of Llyr" and "The High King" (1968); this book became "Taran Wanderer" (1967). The five novels detail the adventures of a young man named Taran, who dreams of being a sword-bearing hero but has only the title Assistant Pig-Keeper. He progresses from youth to maturity and must finally choose whether to be High King of Prydain. Alexander also wrote two spin-off children's books from the Prydain series "Coll and His White Pig" (1965) and "The Truthful Harp" (1967). Alexander won the Newbery Medal for "The High King" in 1969.