Killing the Impostor God: Philip Pullman's Spiritual Imagination in His Dark Materials

I was skeptical before reading this, but it ended up being great and really enhanced my appreciation for the novels. After a slow start, it picks up and is really good by the end. The book also has short plot summaries of the novels, which were very helpful as I read them 2 years ago now. However, it also has an interview with Gregory Maguire which I thought was pointless. I was also unsure of the authors' "agenda" - are they Christian? Regardless, their views were well-thought-out and riveting. I would recommend buying all 4 books and reading this after the trilogy.