The Torn Page is an item from Witchery. It can be found as loot, typically in Bookshops in the chest and in the item frames. It can also be obtained by killing certains monsters (such as Villagers, Zombies or Pigmens) when being a Vampire with the Observations of an Immortal in your inventory.
Torn Pages are used to help advance in Vampirism. Shapelessly crafting a Torn Page with the Observations of an Immortal book will add a single page to the book, which will help define the path to leveling up as a Vampire.