The books do a great job of dealing with dark topics and have loads of perfect tension points for philosophical enquiry. The themes in the two I have used (The Grand Wolf and The Wolf and the Shadow Monster) are death and fear respectively and both prompted fascinating discussions on these topics in years 1 and 2.
The spider character who reoccurs in every book providing a new way of framing the problem to help the protagonist is an especially useful generator of philosophical bite points. For example, in The Wolf and the Shadow Monster she says, "You can change how you feel using your mind". Doing this story with my year 2 class, we broke the circle on mind and then I asked them, "Is the spider right, can you change how you feel using your mind?" It was a fantastic discussion in which the children shared many surprising psychological insights. Our reading of The Grand Wolf facilitated a frank discussion about the nature of death as well as possibly distressing changes in general. After asking the children, "What does it mean, that the Grand Wolf died?", we discussed 'Is it bad when the world changes?' after the book's characters note that they do not want their world to change. This session was heavily praised by the teacher who was moved to shed a tear by the book's ending.
The children have also found the artwork compelling and interesting. I have had a lot of success with these books in the classroom.