For position you never played, book probability is perfectly fine for me. The question starts when you already played position x-times. I'm looking for function that will update book probabilities with your style. Obviously if you play 1.d4 in 97 games out of 100 games, probability of playing 1.e4 runs close to 0%. On the other hand, if you played some late position just once, it's nowhere close to 100% you will repeat the move. I could probably make some workaround myself, but this looks like problem data scientists must have already solved, as it is applicable to virtually any discipline where you want to predict someone's behavior.
Edit: I think there should be one parameter <0;1> measuring how seriously we take book probabilities and how seriously we take players games. With one extreme being ignoring book and one being ignoring player's games.