Magic Tree House by Mary Pope Osborne (ages 6-9)

One summer day in Frog Creek, Pennsylvania, a mysterious tree house appeared in the woods. Eight-year-old Jack and his seven-year-old sister, Annie, climbed into the tree house and found it was filled with books.

They soon discovered that the tree house was magic and could take them to the places in the books. All they had to do was to point to the picture and wish to go there.