House of Many Ways, by Diana Wynne Jones

Charmain is volunteered by her aunt Sempronia to watch her great-uncle William's house while he is away for health reasons. William is a magician, and both he and Sempronia assume that Charmain has basic knowledge of magical things-- but she doesn't, because her mother doesn't think it is nice or proper to learn such things. Charmain is excited about it only because she thinks it will give her more uninterrupted time to read books. The house is a magical one-- it seems to have only two rooms, but if you go through the door to the kitchen and turn just as you go through, you find yourself in a long hallway with the study and several bedrooms, and a window that lets out onto a meadow-- and that is just one of the simpler of the "many ways" of this house. Charmain finds herself adopted by a dog named Waif, and becomes friends with Peter, who has come to be an apprentice to her uncle; Peter is a bumbling magician, but has much more practical knowledge than Charmain herself, who doesn't know how to do laundry or dishes.

Charmain loves books, so she takes her new-found freedom as an opportunity to volunteer to help the king and his daughter with sorting out the royal library. They kindly accept her offer (no one else ever bothered to ask if they might need help!), as they are organizing their books and manuscripts to find out what happened to their once-fortunate kingdom, and how they might recover the fabled Elfgift that kept the kingdom safe.
High Norland, where Charmain lives, is a neighboring kingdom to that of Howl and Sophie and Calcifer, and the King of High Norland invited Sophie to come help them with looking for the Elfgift. He didn't invite Howl because it would be impolite to poach another King's wizard, but Howl and Calcifer were unable to console little Morgan after his mother left, so they all come along-- with Howl in a rather humorous, annoying disguise.
An entertaining, delightful story; it's always fun to see more of Howl, Sophie, and Calcifer (and now the little terror Morgan-- imagine dealing with a magically-inclined toddler!). The story has twists and turns a bit like Great-Uncle Willliam's house, so that one bit of the story ends up turning and connecting with another part in a way that you didn't quite expect, but it still is all rather homey and comfortable. The story is also quite hilarious at times; early on, Charmain decides to try to work a spell for flying she finds in the "Boke of Palimpsest"-- but she keeps having to go down to the kitchen to get ingredients or tools, and every time she looks at the next step of the spell, the page has turned without her realizing it, so she ends up mixing several different spells together.

Title:	House of Many Ways
Author:	Diana Wynne Jones
Date published:	2008
Genre:	Young Adult Fantasy
Series:	follows Howl's Moving Castle and Castle in the Air
Number of pages:	404