Half Magic
By Edward Eager
Illustrated by N.M. Bodecker
1954, Harcourt Brace Jovanovich
Half Magic is a classic of children's fantasy, and it has held up quite well through the years. It is also the first in his loosely-connected Tales of Magic series. It is not necessary to read these in order, however Half Magic is a good starting point simply because it is a very good book.
This is the story of four siblings, Katherine, Mark, Jane, and Martha, who find a magic talisman one summer, and get in a lot of trouble. The talisman, a small coin, grants wishes -- any and all wishes -- but only half-way. If you wish for your cat to be able to speak, it will only be able to half-speak; if you wish for things to go back the way things were (to appease your angry cat), it will now be able to only one-quarter speak.
This would be trouble enough, but the coin will grant (half) wishes to whoever is holding it, even if they do not know that they are holding a magic coin. As you might expect, this causes all sorts of high jinks. It doesn't, however, take long for the children to catch on to how it works, and they mostly clean up their messes, and have some intentional adventures, such as travelling back to Camelot and meeting King Author, before things really fall apart.
Half Magic succeeded as an engaging fantasy because it sets down clear rules for the magic and explores them, something that fantasy, especially children's fantasy, often does not do. Harry Potter, for example, does not really have any rules -- you wave your wand, and things happen. Or don't. By today's standards the magic in Half Magic is a bit childish and playful, but that is saved in part by it being written with a fairy tale overtone, meaning that everything had to come out right in the end, and in part because it was originally written as a 'historical novel' (set, I believe circa 1920), and still works as one 60 years later. It is also saved, of course, by being fun and intellectually engaging, albeit at a 10-year-old level.
The Tales of Magic series is barely a series at all. The next book is Knight's Castle, which tells of the magical adventures had by Martha and Katharine's children, a generation later. However, the series was not published following the internal chronology, and the third book, Magic by the Lake, will return to the children's younger days.