Winter of the ice wizard - Mary Pope Osborne

Merlin the magician and Morgan le Fay have disappeared, and in order to
find them, Jack and Annie journey with their friends Teddy and Kathleen to
the Land-Behind-the-Clouds, a frozen kingdom of ice and snow. There they must
overcome their fears and solve the Ice Wizard's riddle, because if Jack and
Annie don't complete their mission in time, Merlin and Morgan will disappear
forever!

Did you know that there's a Magic Tree House book for every kid?

Magic Tree House: Adventures with Jack and Annie, perfect for readers who
are just beginning chapter books
Merlin Missions: More challenging adventures for the
experienced reader
Super Edition: A longer and more dangerous adventure
Fact Trackers: Nonfiction companions to your favorite Magic Tree House
adventures

Have more fun with Jack and Annie at MagicTreeHouse.com!