The Wonderful Wizard of Oz is part of a series of books written by Man of Letters L. Frank Baum. It chronicled the adventures of Frank's daughter, who was also a hunter, Dorothy, while she was trapped in Oz. According to Dorothy, not everything that her father wrote actually happened, but many of the key events were based on truth. Charlie Bradbury has read the books and was well-versed with their content.
Before Dorothy returned to Oz with Charlie, Sam gave Dorothy a copy of the book.
As the first case investigated in the Men of Letters bunker involved Dorothy, the book has an added significance as an important literature of the Men of Letters. Charlie realized that the book series contained clues on how to deal with The Wicked Witch, left by Baum for his daughter. The clues led James Haggerty to get poppy extract to use against the Witch and Charlie to kill the Witch with the Ruby Slippers.