The Lion, the Witch, and the Wardrobe by C.S. Lewis
Modern Fantasy (Chapter Book)
Siblings Peter, Susan, Edmund, and Lucy are sent to live in the English countryside with Professor Digory Kirke at the begining of World War II. Shortly after they arrive at his house, they decide to go exploring- and find a magical wardrobe that takes them to the land of Narnia. The four siblings explore Narnia and have many adventures there- eventually becoming the Kings and Queens of the land! After 15 years there, they travel back through the wardrobe to find that no time has passed in England. They tell the Professor about their wild adventure in Narnia, and he tells them that one day they will return- but never again through the wardrobe.
Since this book is a little complicated, I think I would use it to improve their vocabulary and check how well they are comprehending. It also opens up an avenue to discuss World War II and the Holocaust due to the fact that the four siblings had to be evacuated from their home. This would undoubtedly come up, and this book would make a great segue.