Will Henry and the great Dr. Pellinore Warthrop have spent the previous two books building their relationship--Will was thrust into the doctor's care by circumstance, but Warthrop is not a paternal, caretaking sort. And yet, as we come into the third book, they are both tentatively aware that they ARE each other's family--that they are becoming indispensable to each other. Yancey even says on his blog that "The books are a love story disguised as a monster story. I can think of no better way to describe them."--and he's absolutely right. For me, this volume was less about the creature they were hunting than the bond between them. 
 Also, Will Henry is headed down a dark path, and I don't think it's one Warthrop is eager to follow him down, so Book 4 had better become a reality. 
 [Audio performance of this series is excellent, btw.]