It’s the second book in a series (and by the looks of things, the middle book of a trilogy), but it reads decently, even if one hasn’t read the first book.  It’s obvious to the reader that there is history among the characters, which might encourage someone who likes this one to go back and read the other, but there’s enough explanation offered so that this book stands okay on its own, without leaving the reader constantly trying to fill holes to figure out what’s going on.  It’s set in late-nineteenth-century London.  The main character, Jem, is a young boy on his own, trying to make his way in the world however he can.  Jem becomes aware that there’s been a rash of children going missing, all within the same neighborhood, and he enlists the help of expert bogler, Alfred, dragging him out of retirement, and offering his services as bogler’s boy, which basically translates as bogle bait.  The team must put themselves in danger and figure out why there have been so many bogles in such close quarters (apparently highly unusual behavior for bogles) in order to save the city from this plague.  It’s engaging, and just scary enough.