The Secret of Secrets is the sixth book in Dan Brown's bestselling Robert Langdon series, published by Doubleday (an imprint of Penguin Random House). Releasing September 9, 2025, this hardcover edition spans 688 pages and delivers a propulsive thriller set in Prague, London, and New York.

Robert Langdon, Harvard professor of symbology, travels to Prague to attend a groundbreaking lecture by Katherine Solomon, a prominent noetic scientist with whom he has recently begun a relationship. Katherine is on the verge of publishing explosive discoveries about the nature of human consciousness that threaten centuries of established belief. When a brutal murder disrupts the event, Katherine disappears along with her manuscript, and Langdon finds himself targeted by a powerful organization and hunted by a chilling assailant sprung from Prague's most ancient mythology. In a thrilling race through futuristic science and mystical lore, he uncovers a shocking truth about a secret project that will forever change the way we think about the human mind.

This hardcover measures 9.5" x 6.3" x 1.2" and features Brown's signature blend of art history, symbolism, conspiracy, and cutting-edge science. The book explores themes of human consciousness, noetic research, and the intersection of ancient mythology with modern scientific discovery.