Tolkien has written several books besides the Lord of the Rings, including The Hobbit and the *Silmarillion. *You could also give him a book such as Pearce/Birzer’s, which explores the very Catholic symbolism in the Lord-of-the-Rings story.
Lloyd Alexander wrote the Chronicles of Prydain, which must be purchased as the separate books (e.g., *Book of Three, The Black Cauldron, *etc.). He has many other books. Similar to C.S. Lewis, Alexander also writes Christian allegorical stories full of moral lessons.
Goldman’s The Princess Bride is a treat; and Ende’s The Neverending Story is a must-read staple of morality tales.
L’Engle’s A Wrinkle in Time.
How about less fantastical but still incredible stories of adventure? Three Muskateers, Man in the Iron Mask, Robin Hood, Count of Monte Cristo and King Arthur in all its various treatments are all great novels, and on a higher reading level.