This is not a typical "series." It's a five-book sequence of fantasy novels written for young people.
Main Characters: Merriman Lyon, Will Stanton
Bold type indicates a book I've read.
Over Sea, Under Stone (1965) [See Review]
The Dark Is Rising (1973) [See Review]
Greenwitch (1974) [See Review]
The Grey King (1975) [See Review]
Silver on the Tree (1977)