Good to see the show is remembering about Sam and Gilly, who were missing for so much of Season 6 as they made their way to Oldtown (via Horn Hill and some Valyrian steel theft).
It looks as though they've really settled into life at the Citadel, including some new clothing more befitting of their setting. Both are studying books, which you'd expect from Sam but also shows that Gilly's learning has come on a really long way. As for what they're reading, Redditor itsjayrr has pointed out the text we see is taken from A World of Ice and Fire, and reads:
“It is also written that there are annals in Asshai of such a darkness, and of a hero who fought against it with a red sword. His deeds are said to have been performed before the rise of Valyria, in the earliest age when Old Ghis was first forming its empire. This legend has spread west from Asshai, and the followers of R’hllor [changed to The Lord of Light in the photo] claim that this hero was named Azor Ahai, and prophesy his return.
"In the Jade Compendium, Colloquo Votar recounts a curious legend from Yi Ti, which states that the sun hid its face from the earth for a lifetime, ashamed at something none could discover, and that disaster was averted only by the deeds of a woman with a monkey’s tail.”
So, is this just an Easter egg, or does it hint that - as many have theorised for years, and Melisandre has long went on about - Azor Ahai will indeed be reborn, and we''ll find out more about who that is (Jon? Daenerys? Hot Pie?) in Season 7?