Magic Tree House Series by Mary Pope Osborne

Magic Tree House is an American children's series written by American author Mary Pope Osborne. The original series was illustrated by Salvatore Murdocca until 2016, after which AG Ford took over. Other illustrators were used for the foreign language editions. The magical treehouse sends Jack and Annie to Nepal on a search mission. . . a ghost! As scary as it sounds, the mission is clear: find ghosts and save the world.

The series consists of two groups. The first group consists of books 1–28, in which Morgan Le Fay sends Jack and Annie Smith, two ordinary children who are siblings from the fictional small town of Frog Creek, Pennsylvania, on various adventures and quests. with a magical treehouse. The second group, called Magic Tree House: Merlin Missions, starts with book 29, Christmas in Camelot. In Merlin Missions, Jack and Annie are tasked by the ancient wizard Merlin the Magician.

Link: https://www.magictreehouse.com/