Blizzard of the Blue Moon, By MARY POPE OSBORNE
Jack and Annie must rescue a beautiful magical creature—the unicorn. But when they land in New York City during the Depression of the 1930s, Jack and Annie are confused. Where will they find a unicorn in a big city?
Formerly numbered as Magic Tree House #36, the title of this book is now Magic Tree House Merlin Mission #8: Blizzard of the Blue Moon.