Season 7 of Game of Thrones picks up where Season 6 left off for many characters, including Sam Tarly, who is now in the Citadel, dealing with a lot of shit (literally) and trying to sneak into the forbidden section of the library.
He's there to find out as much as he can to help in the fight against the White Walkers. So far he hasn't been very successful – or so he thinks. Because there was a shot of a VERY interesting dagger in the book Sam was reading.
Yep, it's the dagger that was used in the assassination attempt against Bran, way back in Season 1.
Catelyn took it to King's Landing, where Varys and Littlefinger examined it, and Littlefinger admitted the dagger was his.
Cat gave the dagger to Ned, and Littlefinger MAY have taken it back when he betrayed him (as he does in the books).