I've begun working on the first TESS story module, tentatively titled "In Enemy Territory." I thought I'd share my process for creating these type of branching-path gamebooks. It's probably not the most efficient way to do it, but it's worked for me on multiple projects.
I work with two different documents--usually a Microsoft Word and a Microsoft Office document, although Google Docs works fine if I want to be able to work from any computer.
I use a spreadsheet to track each section in the gamebook.
The first column just has each section in order. In some of my (as-yet unreleased) projects, I may break the story into different chapters, each with multiple numbered sections, such as Chapter 3.1, 3.2, 3.3, et cetera. In "In Enemy Territory," I think I'll just use numbered sections without chapters--probably between 100 and 150 sections.
Whenever I write and finish a section, I'll highlight it in green. Then I mark in column two which section or section the player can go to from there. For example, section 1 leads to sections 4, 5, or 8.
If a section has a branch that leads to it and has not yet been written, I highlight it in yellow. You can see that section 2 leads to section 6, 11, and 15, none of which have been written yet.
The third column tracks which section or sections lead to a particular branch. Section 1 is the only way to get to section 5, for example. This helps me if I ever need to backtrack--I can quickly follow the trail backwards, all the way to the beginning of the game if necessary.
You can see the first little bit of the story here. Bear in mind that this is an early draft. I have not, for example, figured out exactly how I want to present the encounters. Still, it might be interesting to see the process.