Holmes's non-fiction relates to both his chosen profession and the role-playing game phenomenon. He was a one-time editor of the Dungeons & Dragons Basic Set RPG rule book. His fantasy consists of a series set in a D&D-influenced world, including four short stories and one novel, while his science fiction includes two pastiches of the Pellucidar novels of Edgar Rice Burroughs and the Buck Rogers novel Mordred.

The first of his Pellucidar pastiches, Mahars of Pellucidar was authorized by the Burroughs estate, but it reportedly blocked his follow-up novel, Red Axe of Pellucidar. Ready for publication in 1980, it only saw print thirteen years later in a private printing. A planned third novel in the series remained unwritten.

Other writing projects included two unfinished novels, one a collaboration with Burroughs' son John Coleman Burroughs, whom he helped treat for Parkinson's disease, and the other a "Conan the Barbarian" novel contracted and paid for by Tor Books but later canceled.

Holmes was a regular guest at Burroughs fan conventions such as the Edgar Rice Burroughs Chain of Friendship (ECOF). He received its Lifetime Achievement Award for his Burroughs pastiches at ECOF '93 in Willows, California. He was slated to appear as Guest of Honor at 2004's ECOF Convention in Sacramento, California, but suffered a stroke and was unable to attend. He was a special guest at the June 2005 ECOF in Portland, Oregon.

Fiction

Boinger the Halfling and Zereth the Elf

• "The Adventure of the Lost City," in Alarums & Excursions, no. 17 (Dec. 1976) and no. 19 (Feb. 1977)
• "Trollshead," in The Dragon, no. 31 (Nov. 1979)
• "The Sorcerer's Jewel," in The Dragon, no. 46 (Feb. 1981)
• "In the Bag," in The Dragon, no. 58 (Feb. 1982)
• The Maze of Peril (ISBN 0-917053-05-2) (Nov. 1986)

Pellucidar

• Mahars of Pellucidar (ISBN 0-441-51590-8) (1976)
• Red Axe of Pellucidar (1993- privately printed)
• Swordsmen of Pellucidar (unfinished)

Buck Rogers

• Mordred (ISBN 0-441-54220-4) (1980)

Other

• "The Cenote," in Doppelganger, no. 8 (1987)
• Danton Doring (with John Coleman Burroughs) (unfinished)
• Conan on the River of Doom (unfinished)

Nonfiction

• "Brain Waves and Thought Patterns" in Analog Science Fact -> Science Fiction, v. 69, no. 5 (Jul. 1962)
• "The Educated Flatworms" in Analog Science Fact -> Science Fiction, v. 70, no. 3 (Nov. 1962)
• "The Split Brain" in Analog Science Fiction/Science Fact, v. 93, no. 6 (Aug. 1974)
• "Lovecraftian Mythos in D&D," in The Dragon, no. 12 (Feb. 1978)
• "A Rebuttal to 'The Cthulhu Mythos Revisted'," in The Dragon, no. 16 (May 1978)
• "Lost Civilizations: Fantasy Supplement for Source of the Nile," in The Dragon, no. 24 (Apr. 1979)
• "The Psychopathology of Wargamers: Shrinks and Simulations," in Space Gamer, no. 26 (Jan./Feb. 1980)
• "Confessions of a Dungeon Master," in Psychology Today, v. 14, no. 6 (Nov. 1980)
• Fantasy Role Playing Games - Dungeons, Dragons and Adventures in Fantasy Gaming (ISBN 0-88254-514-0) (1981)
• "Basic D&D Points of View,"in The Dragon, no. 52 (Aug. 1981)
• Basic Human Neurophysiology (ISBN 0-444-00797-0) (with David F. Lindsley) (1984)
• "Mail-Call of Machen Society Cthulhu Meeting," in Crypt of Cthulhu, v. 7, no. 7 (1988)
• "Lovecraft in the Comic Books: an Update," in Crypt of Cthulhu, v. 17, no. 1 (1997)