There is significant doubt about the actual publication of this product
This RPG Item has been moved to (Unpublished Products)
If the product can be demonstrated to have been published (to exist) it then can be appropriate categorized.
Summary of what is known by Shannon Appelcline, copied from his post in January 2019, here: Re: Does This Actually Exist?
Little Solder Games and their immediate successor Phoenix Games published a series of "generic" FRPG supplements from 1976-1978, mainly intended for unofficial play with OD&D. In 1983, Lou Zocchi's Gamescience then released The Fantasy Gamer's Compendium, which collected six different generic fantasy supplements. The Book of Demons, the Book of Monsters, the Book of Shamans, the Book of Sorcery, and the Book of Treasure are all well-documented as being published in the 1976-1978 timeframe. However, the sixth book included in the compilation, the Book of Mystery, seems unlikely to have been published in that time period, though internet listings have occasionally said otherwise.
Instead it seems most likely that Bob Liddil, the author of the Book of Mystery material included in The Fantasy Gamer's Compendium, wrote it original for Lou Zocchi and the Compendium.
The evidence against separate publication in the '70s is as follows:
1. The Book of Mystery is not listed in _Heroic Worlds_, a largely complete listing of RPGs published in the '70s and '80s.
2. There are no pictures of it online, and there seem to be no claims of ownership, or even seeing the game, that hold up.
3. In 1978, Bob Liddil, the author of the material, was just starting to release his own material through his own Rider Fantasy Games. It was all written for the Tunnels & Trolls game, and it's unlikely he had any familiarity with OD&D at the time.
4. Further, Liddil was in Ohio while Little Soldier was in Maryland, back in a time when that sort of distance really meant something.
5. Conversely, Liddil did have a relationship with Gamescience, who started distributing his material in an ad-hoc basis in 1978. He even wrote a book for them, Bones of Power, which was published in 1990, the same year as the revised Fantasy Gamer's Compendium. We don't know of a creative/freelance connection back in 1983 when the first edition of that compilation came out, but it nonetheless seems much more plausible than the alternative.