Game Of Thrones: 15 Plot Threads The Show Totally Abandoned
13. Sam's Books
When Sam left the Citadel - sidenote: Sam is now Grand Maester and still allowed to be obsessed with sex? Helps to be close with the King and his cousin, I suppose - he took with him a number of books. Huge tomes, many of which would probably bore you to death, but some of which were filled with fascinating bits of lore.
It was in those texts that Gilly discovered Rhaegar Targaryen married Lyanna Stark. It was where we saw dragonglass and the Valyrian steel dagger. It was even, in the promo shot above, where there was a mention of Azor Ahai.
Those books are filled with possibilities, including, possibly, answers to so many questions around the White Walkers and Long Night, or at least the weapons that could defeat them, or who knows what else. They're packed away, taken to Winterfell, and then... just, never really mentioned again.