I have actually just finished this trilogy of books tonight. Definitely written for children, but Pullman does draw many adult parallels, especially in the last book. It's more of a single book, divided into volumes, as Lord of the Rings is a trilogy but only in printed format. So if you read one, you will have to read the others.
The story is very good, and I found myself reading all three books in less than a month, which is good for me with my job. When I first started with The Northern Lights I didn't think I would finish it at all! But as the character Lyra develops, as I discovered more about her, as well as daemons, I couldn't help reading on to know her story.
The Amber Spyglass has left me a bit disappointed. The ending is rough, even for an adult, and I'm sorry for kids that read this and get caught up with the two main characters. The ending makes sense in an adult-type of wisdom, but it's a bit sudden and distressing.
One has to respect that it was written for the young-adult to children audiences, and in that perspective it is very well written. All in all, I enjoyed the series, even through my initial skepticism.