Jack and Annie on Ice: Eve of the Emperor Penguin by Mary Pope Osborne
In Book 40 of Mary Pope Osborne's Magic Tree House series, Jack and Annie receive their last charge in the "Merlin Mission" sequence, to discover the final secret of happiness, a mission made more crucial by the serious illness of Merlin himself. Again the two accept the challenge and their mysterious tree house drops them down in Antarctica, near the research facility at McMurdo Station.
Trying to blend into the tour group of scientists and journalists, the two board a bus and then a helicopter which takes them to view the volcanic activity on Mt. Erebus. Glad to see that they have accomplished the "on wheels, by air" portion of their orders, Jack and Annie remained unnoticed among the group until Jack's altitude sickness causes their guide, Nancy, to realize that they are kids. Ordered to wait for transportation back to the base, Annie wanders outside to make photographs, and when Jack follows, the two fall into a deep, snow-covered crevice in the ice and discover themselves floating on an ice floe into an underground colony of penguins, ruled by an Emperor Penguin who is indeed wearing a crown.
However, Jack and Annie feel that they have failed to discover the final secret among the penguins, but they do adopt an adorable orphaned baby penguin and resolve to take him back to Merlin, hoping that the appealing little animal will rouse him from his decline. And their intuitions are right. Annie and Jack learn that the final secret of happiness--taking loving care of someone who needs you--has at last completed their four Merlin missions.
As always, in Magic Tree House #40: Eve of the Emperor Penguin (A Stepping Stone Book(TM)) Osborne skillfully weaves adventure and geographical information into a beginning chapter novel which will please her many readers. Sal Murdocca's illustrations here are of especial value in picturing the esoteric setting in which the author has placed her young heroes.