“BTB” or “By The Book” means different things to different people. In its simplest sense, it's playing a game “RAW”, or “Rules As Written”, where players follow the rules rather than changing or ignoring them.
In the modern D&D world, it's safe to say that BTB players appear to be the minority. The popular mantra is “rulings before rules”. Rules are treated as being unimportant or constraining. DM fiat is applied heavily. Hands are waived. As a result, gameplay will feel less meaningful to some players, who will lack rules they can rely on to judge scenarios and make game decisions. It is “storygaming”, and stands opposed to classic, gamist, challenge-based adventure gaming.
If you're here, chances are you know something about this schism. If not, welcome to adventure gaming!
So, what does “BTB” mean for Demon Idol? It means using the game rules as the foundation of the campaign.
When learning AD&D for the first time, one should strive to play the game rules-as-written. Once mastery begins to accumulate, one will intuitively know when and what rules to adjust to suit the campaign.
Knowledge of another version of D&D does not translate to an understanding of AD&D.
The game must be played in order to understand what rules are good or bad, and why. It is not possible to gain this knowledge by reading forum or blog posts, but these perspectives can be valuable to developing one's own.
When modifying or extending the game rules to suit the campaign and milieu, the spirit of the game must be paramount.
It is not possible to understand the spirit of the game without playing the game properly for some time.
Knowledge of the spirit of another version of D&D does not translate to an understanding of the spirit of AD&D.
An understanding of OD&D lends itself to an understanding of AD&D, and vice-versa. Reading OD&D books, Judges Guild content, and the earliest zines earns one wisdom.
Simply put, when we say “BTB”, we mean “rules first, rulings second, and only in the spirit of the game”.