Prydain is an imagined territory, somewhat like Wales and peopled with characters whose genealogy stretches back to Welsh legend.
The Book of Three takes up Prydain's history during a wonderfully uncertain time -- perhaps at the end of the Dark and the start of the Middle Ages. Mankind was still in the process of "becoming" in Prydain then. For instance, there are two characters here who begin to cross the line into humanity -- Gurgi, a near animal, comic in his cowardice and irresponsibility, who begins the change by responding to kindness through serving with no ulterior motive for once; Doli, a dwarf who left his enchanted underground world behind because he had forgotten the trick of making himself invisible. In fact, the people of the time were forgetting, too. There were still those who could control occult power, but the methods of invoking it were not being systematically handed down; some was lost and some hoarded for evil ends. Taran, young boy, dubbed "Assistant Pig Keeper" to satisfy his dreams of glory, is the central character. A ward of the wizard, Dallben, he is in charge of an oracular pig, Hen Wen. His search for her after a raid by the horrible Horned King brings him to such strong fantasy characters as: Gwydion, a prince who teaches him the first principle of leadership -- self control; Eilonwy, a runaway junior witch, and Fflewddur, an incompetent bard. If these characters don't suggest T.H. White's treatment of the Arthurian legends, they should. The author draw his figures with the same touches of irritability, doltishness and contrariness that leavens with high good humor the high fantasy. The major theme is good against evil-- black magic against white -- but (give thanks for creative restraint) only to a draw.
Hopefully, Prydain's history will be continued.