Book 13 in the Magic Tree House series (1998)
A novel by Mary Pope Osborne
In their first adventure as Master Librarians, Jack and Annie go to the city of Pompeii to bring back an ancient story that is in danger of being lost forever. Little do they know they are saving the myth of Hercules! But before they can find it, the town's volcano erupts in a mighty explosion. Just when things look hopeless, Jack and Annie get some unexpected help from a certain mythic hero - and the rest, as they say, is history.

Book 11 in the Magic Tree House series (1998)
A novel by Mary Pope Osborne
Jack and Annie are whisked off to the vast plains of Africa, where they must solve Morgan le Fay's third magical riddle. But that's only the beginning! Once the riddle is solved, they still have to get past a pride of lions, a humongous herd of rampaging wildebeests, and one very hungry Masai warrior.

The tenth book in the Magic Tree House series (1997)
A novel by Mary Pope Osborne
The saga and success of The Magic Tree House continues! The tenth adventure, Ghost Town at Sundown, is filled with the excitement, action, and fun facts always found in Magic Tree House books. Morgan le Fay has promised to make Jack and Annie masters of the tree house if they can solve four riddles. In Ghost Town at Sundown, the Magic Tree House whisks Jack and Annie back to a ghost town in the Wild West of the 1880s.

The ninth book in the Magic Tree House series (1997)
A novel by Mary Pope Osborne
Morgan le Fay will make Jack and Annie masters of the tree house if only they can solve four riddles -- which will take four books, of course! Dolphins at Daybreak begins the third set of four books in this magical (and increasingly popular) series! Jack and Annie are off in the Magic Tree House again, this time to a whole new world under the ocean.