The Invocation of the Watchers, written by an unknown writer named Lemegeton, is a book that can be found after returning 10 books to Samuel for the quest The Pen is Mightier.... After returning any 10 books to the Brightwall Academy, in addition to The Book of Mysteries, Samuel will give you a quest to find this book in a trapdoor by a house in Bowerstone Old Quarter, and give you a key.