Brennan's Man, Myth & Magic was published by Yaquinto Publications in 1982; he had originally started developing it as a much smaller RPG called Arena, but the game continued to grow from its Roman has as he developed it.:164 Brennan also designed Timeship, the last roleplaying game published by Yaquinto Publications.:165 Brennan wrote several different gamebooks, including GrailQuest (1984–1987), a series of eight Arthurian adventures, and two published only in French – Aztec Quest (1997) and Egyptian Quest (1997).:165
His works have been translated into approximately 50 languages, he has also written books on the Occult and New Age. More recently, his teenage novel Faerie Wars achieved The New York Times best-seller status in the United States and is now part of a five book series. He initially trained in esoteric teachings and Qabalah with the Fraternity of the Inner Light. He later became associated with Dolores Ashcroft-Nowicki and the Servants of the Light.
In 1995 he contributed two novels to the Horrorscopes series under the house pen name "Maria Palmer". In 1998 he published Martian Genesis an ancient astronaut book which claimed the human race is of extraterrestrial origin.
In 2003, Brennan published the children's book The Book of Wizardry: The Apprentice's Guide to the Secrets of the Wizards' Guild under the pseudonym "Cornelius Rumstuckle." The book purports to instruct children on how to become a wizard; it discusses crafts such as making a wand, and includes astrological references throughout. According to the biographical note on the back cover, Rumstuckle "joined the Wizards' Guild in 1514 and became its youngest president ever seventy-eight years later, a post he holds to this day." The novel stars "you" as you train to become a wizard (in wiccan style). You then participate in a "wizard's adventure" a game where you utilise the knowledge you gained after reading the book. The ending is ambiguous, leaving you with the location of the Wizards' Guild, possibly hinting a sequel. In 2011 Herbie Brennan's book The Secret History of Ancient Egypt was republished by Bedford Square Books.
Egyptian Quest (1997)