Jane, Mark, Katherine, and Martha are four perfectly ordinary kids living in a perfectly ordinary Ohio town in the 1920s. They love to read books, particularly the fantasy novels of E. Nesbit, and bemoan the fact that nothing fantastical ever happens to them. Then one day Jane happens upon a perfectly ordinary nickel on the sidewalk, puts it in her pocket, and utterly unwittingly starts the story going.
That perfectly ordinary nickel turns out to be a perfectly extraordinary magic talisman, or, rather, a half-magic one. Because, unlike in all the stories about wish-granting they’d read, the children figure out that this talisman grants its bearer every wish by halves. That is, if you wished you were somewhere else, you’d end up halfway between where you were and your destination. If you wished to turn into a dragon, only the top, or the bottom, or the left or right part of you would turn into a dragon, while the rest stayed the same. The children quickly discover that they can beat this catch by wishing for twice as much as they want of everything, and they go on some marvelous adventures. But of course things aren’t as easy as all that.
My brother and I enjoyed reading Half Magic immensely when we were young, and my kids enjoyed it a great deal as well. As a dad, I particularly liked the fact that the book is set over 80 years ago, and is to my knowledge fairly accurate in its representation of the time, so it has some legitimate educational value. I love the fact that the protagonists themselves love to read books, and real books at that—in fact, I wonder why more books featuring kids don’t mention those kids reading. And I like the fact that the book uses a great many words that most would consider "too big" for most kids, so that kids who read it can either figure the words out from context or look them up, but either way their vocabulary grows without the words being drilled into their heads in school.
The author, Edward Eager, wrote seven books about magic and children, of which Half Magic is the first. My family and I have also read Magic by the Lake, which features the same children as in Half Magic, and we are currently reading Seven-Day Magic. They are all written in the same vein, with a fun mood and a comical touch. The only slight caveat I would offer is that, as the books were written about fifty years ago, there are occasional remarks based on sexism or racial stereotypes, but nothing likely to offend anyone too greatly. NPR has an excerpt from the beginning of Half Magic online.