Amazon has C.S. Lewis The Chronicles of Narnia Series: Books 1 - 7 (Kindle) for $2.99 each.
Book 1: The Magician's Nephew
Book 2: The Lion, the Witch and the Wardrobe
Book 3: The Horse and His Boy
Book 4: Prince Caspian: The Return to Narnia
Book 5: The Voyage of the Dawn Treader
Book 6: The Silver Chair
Book 7: The Last Battle
Book 1: The Magician's Nephew
Book 2: The Lion, the Witch and the Wardrobe
Book 3: The Horse and His Boy
Book 4: Prince Caspian: The Return to Narnia
Book 5: The Voyage of the Dawn Treader
Book 6: The Silver Chair
Book 7: The Last Battle