Demonologist: Book (2013)

A stolen child. An ancient evil. A father's descent. And the literary masterpiece that holds the key to his daughter's salvation. Professor David Ullman is among the world's leading authorities on demonic literature, with special expertise in Milton's Paradise Lost. Not that David is a believer-he sees what he teaches as a branch of the imagination and nothing more. So when the mysterious Thin Woman arrives at his office and invites him to travel to Venice and witness a "phenomenon," he turns her down. She leaves plane tickets and an address on his desk, advising David that her employer is not often disappointed.