I actually met Susan Cooper once, and she is a really interesting, fun lady. This book, and the whole series, are rightly considered to be classics of the genre in many ways. The only things I would bring to a new reader's attention, is that you might want to stop and mull over the feelings you get from reading this book.
The Old Ones, in some ways, are like the ultimate In-Group, the ultimate club. Some of the things that happen, when the Old Ones are around, e.g. the way they can stop time for ordinary mortals while they talk among themselves, can be read as highly realistic depictions of interactions between, say, the popuilar kids in school, when someone of lower standing is in the room. Watch out for stuff like this. The psychological experience of forming an in-group, of knowing secrets that set you apart and above, is what Susan Cooper is setting in stone here. Considered as a spiritual experience, you should remember that this has its drawbacks. For example, if you look at the descriptive adjectives applied to the defenders of the Light, the words "grim", "cold", and "high" come up a lot, but you almost never hear them described as "kind", or "warm". Also, they are locked in a highly abstract struggle -- how about the actual acts of daily compassion or kindness that someone might perform? These seem to get sort of lost, glossed over, because they aren't as imposing or grand-sounding as ideas about the Light and Dark clashing. My feeling is that a lot of this stuff comes out of Susan Cooper's own experiences in a very class-ridden society, and at Oxford University. The ideals in this book are not things that Thomas Jefferson would have supported, to put it mildly. I am being a little facetious here, but I am serious at the same time.
Susan Cooper set out to write an exciting story, and an involving one, and she has certainly done that. I like Susan Cooper a lot, I consider myself very fortunate to have been invited into her home. I love this book, and I know I will read it to my children someday. But for someone coming to it fresh, just try to be a sophisticated reader, while reading this, and ask yourself if you might be being asked to accept things that we might all be better off not accepting. Try to ask yourself whether any of the "magical" ideas in here might be a mythologized way of looking at very real, disturbing parts of social interaction. That's my advice, anyway. Definitely worth reading, but please remember what I was saying about whether these impressive Lords of the Light could be, maybe, a little kinder.