J. H. Brennan (Ireland) is the acclaimed author of over 90 books, both fiction and non-fiction, including Forbidden Truth, The Alien's Handbook, and The Spy's Handbook (all published by Faber & Faber), Death: The Great Mystery of Life (Carroll & Graf), Martian Genesis and The Atlantis Enigma (both published by Dell), The Magical I Ching and Time Travel (both published by Llewellyn), and the popular Book of Wizardry (Llewellyn) under the alias Cornelius Rumstuckle. His works have appeared in more than fifty countries in Europe, Asia, North and South America, and Australia.

Brennan is best known for his young adult fiction series, Herbie Brennan's Faerie Wars (Bloomsbury) with combined sales exceeding 7.5 million. The creator of several role-playing books, his solo fantasy gamebook series GrailQuest (Dell) has sold over 6.5 million copies.

He resides in County Carlow, Ireland with his wife and ten cats.