Mary Pope Osborne - Christmas in Camelot

Book 29 in the Magic Tree House series (2001)
A cup, a compass, a key: the magic tree house has brought siblings Annie and Jack to Camelot, where they embark on a mysterious mission to find these enigmatic gifts. In this special hardcover addition to the bestselling Magic Tree House series, the young adventurers must travel to the Otherworld, an "ancient, enchanted land beyond the edge of the Earth, the place where all magic began" to save Camelot from dark wizard Mordred's evil spell. If they fail, Camelot will be forgotten forever. Fans of the beloved, highly readable series by Mary Pope Osborne will rise to the challenge of this longer, more complex companion to her other titles, which include Earthquake in the Early Morning and Twister on Tuesday. As in every story in the sequence, Jack and Annie bravely plunge into their quest, learning about a culture and time very far removed from their own, and prove once again that children can make a difference. (Ages 6 to 9)