LOTR (including the hobbit) - It took me until the third try to actually grasp all the information in these books by JRR Tolkein. Proud to say I read all the books before seeing the movies. (I've re-read about 7 or 8 times)
HP books - Because duh I was Harry's age when the first book came out
The Stormlight Archive Series by Brandon Sanderson (He still hasn't finished the series.. but I have re-read them from the beginning each time he finishes a new book) - Really good if you like Fantasy
Game of Thrones (Have read them multiple times) And no i refuse to watch GOT on HBO until he FINISHES THE DAMN BOOKS. LOL