The Snow Spider by Jenny Nimmo

From chapters.ca:
On Gwyn's 9th birthday, his grandmother tells him he may be a magician, like his Welsh ancestors. She gives him five gifts to help him - a brooch, a piece of dried seaweed, a tin whistle, a scarf, and a broken toy horse. One blustery day, unsure what to do with his newfound magic, Gwyn throws the brooch to the wind and receives a silvery snow spider in return. Will he be able to use this special spider to bring his missing sister, Bethan, home? The Snow Spider spins an icy, sparely web of mystical intrigue that sets the stage for the next two books in this outstanding trilogy!

The Dark is Rising by Susan Cooper

Written in the style of high fantasy, The Dark is Rising tells the story of the struggle between the forces of the light and those of the dark against an Arthurian and modern backdrop.

Books in the series:
Over Sea, Under Stone, The Dark Is Rising, Greenwitch, The Grey King, and Silver on the Tree.