Mercury, in this book's sense, refers to a Roman and Greek God. In Roman mythology, Mercury is the god of commerce, communication, and travel. He is often depicted with a winged hat and sandals, symbolizing his swiftness and ability to move between realms. In Greek mythology, Mercury is known as Hermes, the messenger of the gods. This explanation suggests that in the context of the book, Mercury is being referred to as a deity rather than any other options provided.