Ghost Town at Sundown

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.
There, they meet a mustang herder named Slim as they search for the answer to
the second riddle.