House of Many Ways by Diana Wynne Jones

A chaotically magical sequel to Howl’s Moving Castle.

Charmain Baker is in over her head. Looking after Great Uncle William's tiny cottage while he's ill should have been easy, but Great Uncle William is better known as the Royal Wizard Norland and his house bends space and time. Its single door leads to any number of places – the bedrooms, the kitchen, the caves under the mountains, the past, to name but a few.

By opening that door, Charmain is now also looking after an extremely magical stray dog, a muddled young apprentice wizard and a box of the king's most treasured documents, as well as irritating a clan of small blue creatures.

Caught up in an intense royal search, she encounters an intimidating sorceress named Sophie. And where Sophie is, can the Wizard Howl and fire demon Calcifer be far behind?