This is sort of a prequel to the first book in the Wayward Children series, although can be enjoyed as a stand-alone and there's very little reference to any other book except at the very end. It revolves around Katherine Lundy, the therapist we met helping the children in book 1, and tells the story of the door she found, and what happened there, and why she left and joined Eleanor West's home.

Lundy's world is the Goblin Market, a wild bazaar type world where everything has a value and everything must be a fair trade, and those who take more than they pay back are slowly transformed into birds. Unlike many of the worlds, those who find the Goblin Market can come and go as they like, up to the age of 18, and they must make their mind up which world to stay in before they turn 18. Lundy's family seem to have an affinity for the Goblin Market; her father had visited it as a child, she encounters it at age 8, and there's suggestion at the end of the book that another relative also finds it. But this story is strictly about Katherine Lundy's, and the friends she makes at the Market. An interesting world and a light fun read like most of the series.