Half Magic, written by Edward Eager.
This is a really magical book that is likened to Harry Potter, but way before its time (published in 1954). The storyline is nothing like Harry Potter's, but the sense of magic and wonder is all there.
Jane, Mark, Katharine and Martha are four bored siblings. It's summer and they're not allowed to do anything fun, like go to the country or a lake. However, they enjoy going to the library and especially love the works by E. Nesbit, which brings them around to discussing magic (on a side note, I love how the author incorporates a recommendation for E. Nesbit's books - on the section about the author, we are told he does this in each of his novels "so that any child who likes my books and doesn't know hers may be led back to the master of us all").
One morning, after the children have finished their chores, they escape outside, and Jane finds a coin on the sidewalk. It just looks like an ordinary nickel and Jane's only thought about it is that she'll think about how to spend it later. And later, when the kids are all lounging around, desperate for something to do, Jane tells them all out of bored frustration that she wishes something exciting would happen.......like a fire! And at that very moment, fire trucks go roaring by. The children follow, and discover that a neighbor's playhouse caught on fire. And the next day, the children's mother, visiting relatives, wishes that she were home instead. Unbeknownst to her, she has Jane's coin in her possession, and no sooner than she thinks that wish, she finds herself halfway home.
By putting together the pieces of the puzzle, the four siblings realize that they are in possession of a magical, wish-granting coin, except only half your wish gets granted. Just by wishing for everything twice or double, you can have exactly what you want! Too bad they don't learn that until after Mark makes his wish, to be on a desert island (they all end up in just the desert). In turn, each of the children get at least one wish on the coin, and the adventures they have are fun & exciting. Even the cat gets involved, and a nice man named Mr. Smith, who comes into their lives in a way they never dreamed.
I recently read this book to the Bachsters and they loved it. My 6-year-old was a little scared by all the magical events at first, but once we got further into the story she loved that aspect of the story. The pace is fast and keeps young reader's attention easily. My only complaint about the book is the scene with the knights battling in the castle - it's a little violent (but pretty tame compared to today's standards). But even that scene, things are repaired & put back in place, so it's really not too offensive.
I highly recommend Half Magic - it's well written and full of magic (well, technically only half-full of magic). And after reading it, I can't wait to check out more books by Edward Eager. And E. Nesbit, too.