The Dark Is Rising Series

The Dark Is Rising is a series of five contemporary fantasy novels for older children and young adults, written by the English author Susan Cooper and published from 1965 to 1977.
Books in the The Dark Is Rising series:
The Dark Is Rising (1986)

On Midwinter Day, his eleventh birthday, Will Stanton learns he is last of the Old Ones - tasked with combating the forces of evil.
Greenwitch (1986)

Jane and her brothers help the Old Ones recover the Grail stolen by the Dark.
The Grey King (1986)

Will Stanton goes to Wales to find the golden harp and awaken the ancient Sleepers.
The cover image shown above is from the 1975 edition.
Over Sea, Under Stone (1986)

The Drew children discover a map in the attic that may point the way to the Grail.
Silver on the Tree (1986)

In the final volume of the “Dark is Rising” series, Will, Bran and the Drews search for the crystal sword.