Easy way would be to build an RNG system into the gamebook, such as "you must enter a 3 digit pass code" and have whatever number is the first digit 'seed' the

Easy way would be to build an RNG system into the gamebook, such as "you must enter a 3 digit pass code" and have whatever number is the first digit 'seed' the RNG options. then later you can ask the reader to validate whichever of the 3 digits, and if they get it 'wrong' it acts as a seed update for new choices.