A great book, though it does have some magical elements and some other spiritual content.
Plot
11-year-old Will Stanton is the seventh son of a seventh son, which means he has been accepted into a darkness-fighting alliance called the Old Ones. He must help to find the four things of Power which will help him defeat the dark and find the six signs.
Morality
The Dark Rider masquerades as a deceptively good guy once or twice, but really, he's pure, pure evil.
Spiritual Content
There's a rather magical feel to the story, due to the fact that Will is transported back in time, it seems, but there's no use of magic or sorcery, specifically. Will and the other Old Ones do use powers of the mind. One scene that takes place in a church is either extremely incorrect or very profound; it possibly suggests that there was a time when God did not exist. Another quick, easily-missed comment mentions something existing "fifteen million years ago."
Violence
The Dark Rider is sent to kill Will, sometimes capturing one of his siblings to get one of his signs. Will sprains his ankle. Other slightly medieval and fantastical violence occurs throughout the book, but it's all rather mild.
Drug and Alcohol Content
Adults drink wine and there are one or two references to alcoholic Christmas beverages.
Sexual Content
Some of the Stanton brothers have girlfriends.
Crude or Profane Language or Content
"Oh Lord" is used several times, as is "Oh gosh."
Conclusion
A climatic, heart-pounding, enchanting book. Susan Cooper is a great author, almost like J.R.R. Tolkien in her vivid descriptions and well-developed characters. This is probably one of my favorite books of all time.