The Hollow Boy, by Jonathan Stroud

I absolutely loved The Screaming Staircase and The Whispering Skull, the first two installments in Jonathan Stroud's horror/adventure series Lockwood & Co. The third book in the series is more problematic than the first two, but just as delightfully spooky.

The Hollow Boy picks up a few months after the events of The Whispering Skull. Thanks to their recent high-profile cases, the small ghost-hunting agency of Lockwood & Co. is now legitimately busy—so busy, in fact, that Lockwood decides to hire an assistant, the hyper-efficient Holly Munro. Lockwood agent Lucy Carlyle isn't thrilled about the perky, pretty new hire, but she has a bigger worry than losing her spot as the only girl at the agency: she wants to try to communicate with ghosts, but her co-workers are justifiably concerned that her experiment will get her killed.

Unlike The Whispering Skull, The Hollow Boy feels like a middle book, with all the accompanying pleasures and frustrations. It deepens readers' understanding and appreciation of the characters and the world, but the storyline raises more questions than it answers. Don't get me wrong; this is still a Grade-A character-driven adventure story, but if Mr. Stroud doesn't start resolving at least some of his plot threads in these middle installments, the final book in this series better be a million pages long.