The Twilight of Magic by Hugh Lofting

Here is a glamorous tale of castles, kings, and cavalcades of knights, of princesses, peasants, and wise women, which gives a vivid picture of the Middle Ages, when adults and children alike still believed in magic.

This starts off as a pretty solid children's fantasy in the classic vein (think a less clever and humorous Nesbit, in a medievalish setting). Giles and Anne are nine-year-old twins, the children of a prosperous wagon-wright who, we learn at once, is inexplicably in debt. They decide to seek help from Agnes the Applewoman, even though most of the townsfolk think she is an evil witch because she has cats and psychic powers. They get a Magic Object and have some low key episodic adventures, including a harmlessly haunted inn and a chat with an alchemist.