So, your question is on how to write a branching story, and have the game be consistent with the end choices that are made by the player?
Sorry, had to read your post a few times, either it's me, os something about it is confusing, I think it might be both the lack of detail, and the inclusion of side info that may confound the core question.
So, if that is indeed your question, you have a few things to consider before you go and tackle this.
Let me make sure I'm getting the basic trajectory
A. You want to write a series of 7 books
B. you want to base a single game of this said series
C. you want it to be narrative intensive, and have branching dialog trees, which will effect the outcome of the story, and ergo, the game.
D. The books & the game are not yet satisfactorily fleshed out on paper yet.
If those hold true, then the specific details of your actual narrative and story would actually just clutter your core question.
Sounds like your trying to plan out this entire undertaking, but the issue is your trying to planning 2 different projects right now.
The books
The Game
Each requires a different mindset, each requires a different narrative structure.
Your trying to jump to the game based on the books, when the books haven't even been written yet. what's that saying? the horse before the cart?
The question you should ask yourself if you really want to have both books and game, is how will the story be different in each?
A question so far ahead, that there's no way of answering it.
Your assuming if you write the books, game adaptation will happen fluidly.
A writer friend once told me she preferred books to all other forms of media, because nothing could beat her imagination.
When you’re just writing, you need to play on that, but when you’re making a game, it's up to you as the writer to communicate to everyone else (mainly the artists), what things could look like, not how it should look. (unless you’re paying them up front to carry out your specific vision).
The power of a writer is not the same as in all other forms of media, it's much more fluid in games. Even if you’re doing a narrative based game, your job then is to map out every branching dialog tree, and every sub branch, as many as you need, but it becomes the coders' job to make that actually work in game.
Anything you write has the potential to become hours, months, or even years of work for the developers, so you need to be very careful about how far you go, the farther you go, the more work, more time, and more motivation you and the team will need to just get to a playable demo.
Take a look at :
https://en.wikipedia.org/wiki/Deus_Ex:_Human_Revolution
That has already staggering dialog trees and choices.
Sorry due, but from the information you gave us,
my best advice is to decide which you want to write more, a book series or a game, trying to plan both at the same time will require more than just an author/ script writer.
Not to mention, unless you’re working for a AA studio, script writing won't help you for conventional game writing, it's too long and drawn out, and most of the time, you don't have time to present your player with a 10+ page story.
But if you still want to do this, just know, if you want dialog choices that matter, they will need to be categorized and standardized across all choices, if you want both, but if they're not meaningful, they become frivolous, and a waste of design and effort.
I don't mean to rain on your parade, but know what you’re potentially getting into.
As the above poster said,
Defining the narrative relationship between the books and the game will allow you to decide where to start.
The best way I'd see this working, is you write and finish all the books, then make a game that highlights key decision points in the books, allowing you to explore critical road not taken paths, it becomes a "what wold have happened if" kind of experience, than an "explore the established story line"
The question becomes which you want more?
Player Agency?
or
Story line coherence/ consistancy
You also might find this interesting:
https://religionannarbor.wordpress.com/2011/02/03/if-video-games-were-invented-before-books/
And
http://www.stanleyparable.com/
