Is my game, Into the Woods..., the kind of thing you are looking for? All the things you encounter are basically fixed stat blocks that you need to roll against. It has two systems in play:
- Target Number - Where you just need to beat the target number with a D6 roll + modifier
- Opposed Roll - Where you need the higher overall number on a D6 roll + modifier
Rewards are given out depending on what phase of the game you are in, and are listed on the creature's stat blocks. So to me, that sounds close to what you are looking for at least. The game has been performing very well from what I can tell (compared to other TRPG's and such), so the system seems to have some decent appeal. My game was just a jam project, so it could certainly do with further polish, but it should serve as a solid base. I have an expansion for the game that fixes some things in the base game while expanding some of the content (at a small cost, base game is 100% free). Let me know if you want to look at that, we can probably work something out.
Anyway, I hope this was helpful. If this wasn't the kind of thing you were looking for, then sorry for the unnecessary reply. I'm interested in learning more about what you are making. Being totally honest, I'm not quite sure what you mean by "gamebook-oriented rule system", but it sounds cool nonetheless. Happy gaming, and in this case, creating! :)