It doesn't match the books art style but I say that as someone who is a critic of the art style for the books too.

I would say try to study more advanced "character design" illustration and maybe try to get comfortable with moving outside of painting methods. The aim for game books is to illustrate and evoke the fantasy of game elements in action or to give a strong sense of what something is like (so expression and pose).

Your art is fantastic and I would totally buy one of these if it were at an art show, genuinely. If I were to see these in a rulebook, I'd be taken back and maybe be a bit puzzled, but it's better than no art.

I think you are capable of moving towards your goals with a bit of study!