It is many years since I have read these books, but both series - Lloyd Alexander's Chronicles of Prydain sequence and Susan Cooper's Dark is Rising books - are excellent, and could give the Harry Potter novels a run for their money.
The Chronicles of Prydain novels were originally published in the 1960s but I discovered them in 1973 when I first came across The Book of Three in an Armada Lion paperback edition along with its sequel The Black Cauldron. They tell the tale of Taran, assistant pig keeper to oracular pig, Hen Wen, and how he is forced into a dire adventure by her disappearance. The series continues in The Castle of Lyr, Taran Wanderer and The High King, the last three of which I have in Dell Yearling paperbacks.
Susan Cooper's Dark is Rising series is highly regarded though I'm afraid I can't tell you much about them as I have forgotten the story, but the blurb at the back of the first volume Over Sea, Under Stone indicates that the story begins with "the discovery of an ancient map and a search for a buried Grail." The story continues in subsequent novels, The Dark is Rising, Greenwitch, The Grey King and Silver on the Tree.
All, except The Grey King, are Puffin paperbacks. The Grey King I bought in hard cover, no doubt impatient for the story to continue.