Editor's Note: In the US, the first volume of the trilogy is called The Golden Compass. Also, a film has been made.
I heard these books described as a thinking-person's Harry Potter. That doesn't really do them justice. The format, the intent, the audience, the world-view, all are different in several degrees from Harry Potter. The main thing is that the Harry Potter stories are in the familiar reductionist mode of children's fiction that has borrowed from the highly stylized world of Saturday morning cartoons. Utterly absent are moral nuance, and pleasures are simple and greedy, even for the "good" characters. Everything is very easy to identify as good or evil, fun or boring, giving feelings of pride or shame, and the violence is strangely sanitized. The only moral choices the "good" characters face is whether to act loyally to their friends ("friendship" being the highest imaginable "good" in this world-view) and the power of loyalty to friends neutralizes any other action, from being silent and stubborn to authority figures, right on up to remorselessly killing "bad" characters.
Pullman's stories, though, apart from seeming to me to be too intellectually challenging for most modern pre-teens (I don't mean that to sound as insulting as it probably does, but really they are quite philosophically rarefied in parts) are more in the old style of children's literature, being as weird as Alice in Wonderland and as violent as Grimm's fairy tales and yet as modern as The Little Prince was in its day. And talk about moral nuance, hard choices, and ethical grey areas, these not only proliferate as the story goes on, they more or less drive the entire plot. There are parts that are better than others in the books, and this is something that I imagine young readers will find especially difficult, because there are times when as the focus switched from one group battling unspeakable evil to another, I was quite tempted to read ahead to where the dropped thread was picked up again, and got quite annoyed at almost turgid parts, even though I could tell they were necessary for the whole picture. The sweep of the stories are quite broad, encompassing universes and their creation and destruction, yet somehow not in the old SF Dr. Who kind of way ("Oh, well, when you've seen one megalomaniac who wants to rule the universe, you have pretty much seen them all") but more in that vertiginous way one feels if one allows oneself to really think about the possible implications of quantum physics.
To sum up the trilogy, without injecting any narrative or teasers, I would say that these three books comprise a Faustian fairy tale about parallel histories, theology and physics, featuring a pair of protagonists who are a twist on the old Tristan and Iseult mythology. And it's got talking warrior bears and witches and magical implements that can be replicated in a physics lab.