there are five books in this series by susan cooper:

1) over sea, under stone
2) the dark is rising
3) greenwitch
4) the grey king
5) silver on the tree

they all centre around the three children of the drew family, or around a different little boy named will stanton, and their collective struggle to save the world from the growing forces of darkness. the struggle feels important without being preachy and is steeped in welsh and english mythology and tradition. it is more serious and meaningful than harry potter without being heavy. i just reread the series last year to my step son (who was 11 at the time) and enjoyed it as much as i did when i was a kid.