Herobrine's daughter scarred me (story time)

so, while I was playing Minecraft single player, like I always do, after awhile, I logged off. When I logged back on, I saw this book in a chest, I was confused, so I read it. The book said: 'I see you _Unknown_.' was shocked, knowing that I was playing Minecraft by myself, so I just quitted for half an hour, then it was gone from my inventory. I still don't know what happened. :/