The book "Satan's Invisible World Discovered" was authored by Scottish preacher and academic George Sinclair. The inquiry in the book looks at the alleged presence of witches, devils, and other paranormal beings in 17th-century Scotland. Sinclair bases his assertions regarding the presence of witches and demons in Scotland on a mix of biblical texts, historical documents, and first-hand testimonies. He gives instances of purported witchcraft trials and executions and goes into great depth on the various means by which witches are said to harm others, such as spells and curses. The book also explores the theology that underpins evil's existence and Satan's place in the universe. Sinclair contends that the presence of witches and demons is evidence of Satan's power and that a steadfast Christian faith is necessary to fight it.