Edward Eager's Seven-Day Magic
Eager's books were some of my favorites when I was a kid (and still are, really)--I think they laid the groundwork for my love of books where magic comes into the real world. I've been waiting and waiting for Kindle versions to come out (my copies are pretty worn) and was pleased to discover last week that three were available. Of course, they're a motley assortment; Eager wrote seven kids' books, and these three aren't connected at all (four of his books take place in the same universe, two in their own, and this one is a stand-alone that sort of relates to the first four). Anyway, my point is that all of them should be available.
If you were curious, this one is about a group of kids who check a magical book out of the library, except it's way funnier than that description makes it sound.