The Hall Family Chronicles is a children’s metaphysical fantasy book series that focuses on the Hall family of Concord, Massachusetts. The Halls are introduced in the first book, The Diamond in the Window, which is the best-known book of the series. In that book, Eleanor and Edward Hall, a pair of orphans, live in the old family home, a large but somewhat rundown and shabby house in Concord, with their aunt and uncle, who are a pair of unmarried siblings. They are in danger of losing their home because the family doesn’t have much money, and they owe too much money in back taxes.

The children’s uncle was once a famous literary scholar, specializing in the Transcendentalist literary movement based in Concord, but he has not worked in years because he is not in his right mind. He hasn’t been the same since the mysterious disappearance of the younger siblings that he and his sister were raising, Ned and Nora. Edward and Eleanor are named after their missing aunt and uncle, and they learn that Ned and Nora disappeared from their beds suddenly one night years before.

At that time, their uncle had a protege living in the house, a wealthy prince who had been in love with their aunt and who was fond of young Ned and Nora. The prince, Prince Krishna, used to set up games and treasure hunts for Ned and Nora with real jewels as the prizes, and it seems that they were working on one of his treasure hunts at the time they disappeared. The prince himself disappeared soon after. Edward and Eleanor find the clues that Ned and Nora were following, all based on Transcendentalist themes, and they decide to try solving the puzzles themselves to find the prince’s treasure and save their home.

It turns out that the prince and Ned and Nora are trapped by evil magic created by the prince’s wicked uncle. Edward and Eleanor begin experiencing it when they start sleeping in Ned and Nora’s old bedroom and following the clues left behind by the prince. They begin experiencing strange dreams in which they see Ned and Nora and the answers to the riddles, but they are in danger of becoming trapped themselves as they approach the final dream.

They do manage to defeat the evil magic and free the prince and Ned and Nora, and they save the family home. When Ned and Nora reappear, they are grown up, even though they were children when they disappeared. Their uncle regains his right mind, and he and the prince open a school for studying Transcendentalism and write a book together. The prince renounces his role as ruler of his kingdom so he can marry the children’s aunt and stay in Concord.

Later books include friends and other young relatives of Eleanor and Eddy as main characters, like their step-cousin Georgie. Many other other reviewers seem to think that The Diamond in the Window and the other early books in the series are better than the later ones.
The series is by Jane Langton.
Books in the Series

The Diamond in the Window (1962)
Edward and Eleanor search for a lost treasure and learn the secrets of the disappearance of their long-lost aunt and uncle.
The Swing in the Summerhouse (1967)
Prince Krishna has to leave on a trip, but he gives the children strict instructions to never go through one of the openings in the summerhouse.
The Astonishing Stereoscope (1971)
The cards for Eddy’s stereoscope show some strangely familiar scenes and a mysterious rope that tempts the children to tug on it.
The Fledgling (1980)
Little Georgie believes that she can learn how to fly, and one night, she climbs onto the back of a Canada goose, to fly away with him and learn how to fly herself.
The Fragile Flag (1984)
When the President of the United States discusses the construction of a nuclear bomb that would be capable of destroying the entire planet, Georgie gets Eddy and Eleanor to march to Washington DC with her to convince him to stop, bringing with them an old flag that can give people magical visions.
The Time Bike (2000)
Prince Krishna gives Eddy an old bike that can travel through time.
The Mysterious Circus (2005)
The Halls get a visit from a family friend which leads to an amazing circus.
The Dragon Tree (2008)
The Hall children try to stop a man who seems determined to cut down the town’s trees, especially a mysterious, fast-growing tree.