*Spoilers and Speculation Ahead*
The episode is called Book of the Stranger and it will most likely focus on the Faith of the Seven, the High Sparrow and the Faith Militant as the title seemingly refers to a section in the faith’s holy text- the Seven-Pointed Star. In the books, Septon Meribald mentions to Brienne about a section in the holy text called ‘The Maiden’s book’. If Maiden, one of the seven aspects, gets her own book, it is possible that other six also do and therefore the title is called Book of the Stranger.
The Stranger is also an aspect of The Seven and though he is referred to as male, he is not associated with any gender. He represents death, the unknown, and also leads the dead to the other world. The Stranger is described as half -human, with his face hidden beneath a hooded mantle. Followers rarely pray or seek favor from him but outcasts are known to associate themselves with the Stranger. Silent Sisters (who shaved Cersei’s head before the Walk of Shame) are sometimes called the ‘Stanger’s wives’.
The Hound named his horse Stranger and the episode may also feature his return. Ian Mcshane has almost confirmed that he’s bringing back Sandor Clegane. Since Stranger is associated with the dead, we might also see Jon Snow’s return in this episode.
All this is highly speculative and we will probably get a better idea once the next 2 episodes air and we get the official synopsis from HBO.