Magic Tree House - A series of really fun chapter books full of adventure that explores all over the world in all different time periods. My kids love them and learn from them and there are also research guides that provide extra information about the subject of the individual books. These books are fun to read and easy to learn from.
Also on these boards
Magic Tree House Tonight on the Titanic Literature Unit
Thanksgiving on Thursday - Magic Tree House Book - Mary Pope Osborne
Magic Tree House: Thanksgiving on Thursday