Magic by the Lake

If Jane and Mark and Katharine and Martha had stopped to think, they might have ordered magic by the pound, or by the day, or even the halves - the way they had known it before in a book called "Half Magic." But they didn't stop to think and as a result find themselves with a whole lakeful of magic. Extraordinary and unexpected things happen as they learn to tame the magic with the unwilling assistance of a sly old turtle. Eventually, in an effort to help their stepfather, Mr. Smith, save his failing bookstore, the children find themselves involved with Ali Baba's forty thieves. By the end of the vacation the children really are able to help Mr. Smith - and in a most surprising way.