The Dark Heart of Time

Set in October 1918—during Tarzan's search for Jane—the novel takes place between Tarzan the Untamed and Tarzan the Terrible.[4]

The novel's antagonist is James D. Stonecraft, an American oil magnate who believes that Tarzan knows the secret of immortality. Stonecraft hires hunters to track and capture Tarzan for the secret, leading to conflicts at the "City Built by God" and the "Crystal Tree of Time". Through all of the adventure Tarzan is focused on escaping his pursuers so that he may return to his search for his wife.[5]

Before The Dark Heart of Time, Philip José Farmer wrote a number of books based to some extent on Tarzan.

• Time's Last Gift (1972) being a time travel story, the novel is both an unauthorized sequel to the entire Tarzan series, and a prequel to the authorized Khokarsa series.[4]
• Tarzan Alive: A Definitive Biography of Lord Greystoke (1972/2006)
• The Adventure of the Peerless Peer (1974) an unauthorized work in which Tarzan meets Sherlock Holmes.[4]
• Khokarsa series, novels authorized by ERB, Inc. set in Tarzan's Africa 10,000 years ago.
  • Hadon of Ancient Opar (1974)
  • Flight to Opar (1976)
• The Secrets of the Nine series, an alternative interpretation of Tarzan and Doc Savage as half-brothers battling a world-wide conspiracy headed by a group known as The Nine.
  • A Feast Unknown (1969) ISBN 0-87216-586-8
  • Lord of the Trees / The Mad Goblin (dos-a-dos Ace Double, 1970) ISBN 0-441-49252-5
  • The Empire of the Nine (Sphere, 1988) Omnibus reprint of the Ace Double with The Mad Goblin retitled as Keepers of the Secrets.