The Magic City
by Edith Nesbit
When young Philip Haldane builds a game city with possibilities and endings, the fantastical creation comes alive and he and his new half-sister Lucy are magically transported there. Now they must try to save the Magic City by fulfilling an ancient prophecy, despite a mysterious enemy who decides to steal their glory for itself. The way Philip and Lucy come to make friends and overcome the impending disaster together makes you read the riveting.