This is a GREAT supplement to "Little Fears, Nightmare Edition."
Many other rule books offer "more classes" or "more perks" or even "more gear." The whole bigger, better, stronger, and faster kind of idea.
This book doesn't do that. Instead, it grows the world of Little Fears. It adds more creepy places and creepy scenarios. Scary places that are neither closet land nor the real world. Among the Missing gives the players a chance to play (or interact with) one of the "Missing." And it gives rise to different motivations and different types of monsters.
A big worry was that this book would treat the idea of missing children either in a fantastic or kind of flippant way. It doesn't do that. This book treats the concept of "The Missing" with respect and the touch of sadness that it deserves. Reading the stories are heartbreaking.
Little Fears is a game based on imagination, fear, and hope. This book just adds another layer to each of them.
If you like Little Fears, Nightmare Edition then I would recommend this supplement. It makes the world larger than just your Cul-De-Sac