The book follows Lucas Corso, a book detective who is hired to authenticate a rare manuscript by Alexandre Dumas. As he delves into the investigation, he finds himself entangled in a mystery involving two other books: a rare edition of "The Three Musketeers" and a satanic text, "The Book of Nine Doors of the Kingdom of Shadows". The narrative becomes a complex puzzle as Corso uncovers connections between the books, while also dealing with a seductive woman who seems to have stepped out of a Dumas novel, a mysterious killer, and a group of bibliophiles obsessed with Dumas. The lines between fiction and reality blur as he gets closer to the truth.

"The Forbidden Kingdom" is a historical fiction novel that intertwines the stories of two men, a 20th century sailor and 16th century Portuguese poet, Luis Camões. The narrative alternates between the two, exploring the sailor's obsession with the poet's life and the poet's experiences in the East, including his journey to Macao. The book delves into themes of identity, obsession, and the clash of Eastern and Western cultures.