From Amazon.co.uk:
Millipop Klompet and Ernest Perriclof live in the uneventful town of Drabville, where the cautious Ernest spends his time enlarging his rock collection and Milli dreams of adventure. When the pair are kidnapped from their homes and brought to live in the gothic mansion known as Hog House, they are adopted by the madcap Mr and Mrs Mayor and a series of bizarre encounters follows. Things do not add up and the children find that sinister plans are beginning to emerge. Why has the notorious Lord Aldor stolen the shadows of all of the town's residents and where are they being held? Assisted by a band of prisoners, the children must venture into the taboo territories, and battle the perils dished out by the Lurid Lagoon, in order to outwit Lord Aldor and prevent him from executing his secret master plan.

Left at the book exchange stand near Factorie and Converse on level 3.

Release #16 for the 2019 The The Challenge.