I'm probably five times older than the target audience for this series, but I'm enjoying it immensely. The protagonist is well drawn as a child who is precocious, not intellectually but socially. That's a switch.
The other characters are well drawn, too. Human or not, you get a decent sense of who or what they are. Their actions are not arbitrary.
The plotting is well done, too. There's enough action, danger, and suspense to satisfy. This is no fluffy fantasy. People are sick, they get hurt, and they die. At the same time, it shouldn't be too much for a tween to handle. They can follow the protagonist as he learns to deal with adults and adult problems such as loss and betrayal.