I read this to my 14-year old son, who gives it 4 stars. My rating is close to 4.5, but I've decided to start rounding up on those so that I can more easily find the books that I've really liked. 
 Happily, this series brings more of the stuff that I loved about the other two, but mixes it up enough that it doesn't feel like a simple rehash. This time, a new player (Holly) has joined Lockwood & Co., and Lucy, ever the narrator, stews in tension over this for most of the book. I liked their bantering and interplay, yet I can see how one would think of it as unnecessary drama, and it was likely the part of the book that my son liked the least. 
 Plenty of ghost-squashing goes on, with not only Lockwood and George (and Lucy and Holly, of course), but several of the kids from other agencies return in this book. Don't hold your breath waiting for the appearance of the Hollow Boy, however. His appearance doesn't occur until closer to the end. That's fine, for the mystery is unfurled slowly and deliciously, thanks to Stroud's superb writing. 
 My son and I both agree that our favorite part of the book is the Skull. Yep, the character we like the best is a brutally honest, wise-cracking smart ass skull-in-a-jar that only Lucy can hear. I didn't shelve The Hollow Boy in my Humor shelf, though this book shares the same wit that all of the Stroud titles I've read exhibit. It helps to break up the tension with the ghost-tromping, as well as any tensions between the characters, and I need Stroud to write another in this series and keep up the funnies! Please someone make a movie on this series, and take great care when you cast the skull.