Why is this a pick?:
Eric, Julie, and Neal are best friends. One day, they discover a flight of rainbow-colored stairs in a basement, leading to a magical world they never knew existed. Before they know what happens, they are racing to save Droon from the evil Lord Sparr. The first book in "The Secrets of Droon" series, 7-10 years. Books stand alone, but form a story arc when read in sequence.