The story is about a girl named Charmain who's only pleasure is reading, and somehow she's forced to housesit for a sick Great-Uncle, who is actually a wizard. His house is quite unusual. At first glance it would seem like the house only had a kitchen and a living room, but if you go through a door and immediately turn left, it takes you to the bathroom and bedrooms. There's also a sweet little white dog that the Uncle left behind, called Waif, who likes to eat a lot. Eventually Charmain is joined by a boy named Peter, who was supposed to be her uncle's apprentice before he got sick. So they're stuck together and take care of the house. Peter is a big help, since Charmain knows next to nothing about doing dishes and laundry and other boring chores. Still, for some reason she finds him annoying.
In the middle of taking care of her Uncle's magic house, Charmain becomes assistant librarian to the King, so she plays a part in solving a mystery that Sophie (remember her from the first book?) is trying to solve, along with her friend Calcifer the fire demon, her husband Howl, although he's disguised himself as an adorable young boy with a horrible lisp named Twinkle (no idea why 0,o) and their 2 year old son Morgan, who I think is something of a brat. I'd rather not reveal what that mystery is, since it would take too long to tell.
If this book was made into a movie, I wouldn't mind at all. The characters are lively with full personalities, and the adventures that Charmain had quite held my interest. I liked how Charmain could sort of do magic, like Sophie found she could in the first book. And the ending panned out a TON better than Howl's Moving Castle. Sure, it was just as much a happily-ever-after as before, but at least everything made sense and all the ends were neatly tied up. If you could get any enjoyment out of this series, I'd say watch the Hayao Miyazaki film of "Howl's Moving Castle," and then read this book.