The Lion, the Witch, and the Wardrobe by C. S. Lewis (1950), followed by six more books in the Chronicles of Narnia series.
The Borrowers by Mary Norton (1952), followed by four more books in the Borrowers series.
The Fellowship of the Rings by J. R. R. Tolkien (1954), followed by two more books in the Lord of the Rings trilogy.
A Wrinkle in Time by Madeleine L’Engle (1962), followed by three more books in the Time Quartet.
Charlie and the Chocolate Factory by Roald Dahl (1964), followed by the sequel Charlie and the Great Glass Elevator.
The Owl Service by Alan Garner (1967).
The Last Unicorn by Peter S. Beagle (1968).
Watership Down by Richard Adams