: How can I convert a linear narrative into a branching narrative? Suppose I would like to create a text adventure game, but my imagination is not sufficient for creating a whole game. But there

Think of it as a braided river rather than a branching tree

Imagine a braided river channel. You can put a rubber duck in at a single point and follow its progress down the river. It has many different possible paths, but it is not locked into a unique path by one early decision. Reliably at many points those paths converge. As it approaches its destination it may have a delta with a few different branches, and only then does its final path to the end of the river become fixed.

Pick-your-path narratives benefit from having a similar structure. Your protagonist starts at a particular scenario and is offered a few decisions, and they come out at one of several possible endings, but along the way they can get to specific scenes from a variety of previous paths. You do not have to write 3^15 different stories, where at every decision you create new paths and scenarios, because you can write decision-making options at several different scenes that feed into a single new scene.

Since you're adapting a linear story, I would suggest doing the following:

Decide what the starting point is.
Consider how you want to handle the novel's ending. (More below.)
Identify any scenes or events in the book you absolutely want your protagonist to experience. These can include character- and world-building moments as well as those that serve the plot. (These will be confluence points for all possible narratives. The fewer you have, the more your paths can diverge from that in the book.)
Map these out in a visual diagram or flow chart. (You don't necessarily have to decide what all of the endings are yet, but include all the ideas you have now.)

Now you can fill in the gaps with as many meandering and braided shorter paths as you'd like. To make the most of the source material, I would suggest trying the following:

Identify any scenes or events in the book you would like your
protagonist to experience that are not absolutely essential to
reaching the points identified in #3 . See if you can create or
identify various smaller, independent sequences of scenes from these
connected by 1, 2 or 3 decisions made in the book. They can overlap. Map these out separately from your
core chart, and include the decisions that connect them. (I.e.,
begin to make small braids that can serve as building blocks.)
Identify all of the illustrative interlude scenes that serve
character- or world building but don't involve decisions that affect
the plot. Hang onto these.

Now you can start creating your own content:

Go back to #5 . For each of the decisions you identified, create 1-3
alternative choices the protagonist could make. Can these lead to
other scenes that already exist? If yes, great! Keep them. If not,
try to create new scenes with decisions to make that can lead back
into existing scenes, especially the ones identified in #3 .
Start putting these smaller braids into the core map in the most
obvious locations. Connect them to the confluence points with new
decisions. Remember that because this is choose your own adventure,
these aren't locked into a chronological order. For example, even if
most choices leading to the braid happen before a specific
confluence point, other paths can conceivably go through these from
after reaching that confluence point (see #3). You can prevent loops
with conditional choices, e.g. if the protagonist has already met a
troll at a confluence point, there is a fallen tree blocking the one
path that would lead back to the confluence point with the troll.
Make sure that none of the paths skip the essential confluence
points.
Hammer out your endings, and make decision chains that bring in
existing braids or entirely new braids that get the protagonist from
the final confluence point to each ending.
If there are any loose ends (decisions that don't lead to an
existing scene), you can start filling those in with original
content. Go back to the scenes identified in #6 for inspiration--
see if you can include these in the path and create related
decisions or actions that can can lead to one or more existing
scenes. (Again, make sure that none of the paths skip the essential
confluence points.) If some just don't work, prune them.
If you have any leftovers from #6 that you still want to use, you
can work these into existing scenes or make them brief interludes
without decisions to make.

Determining your endings

You'll want to decide from the beginning roughly how many ending options you want. This may hinge on the way the novel is structured and the sort of conclusion it reaches. Some possible structures:

Does your protagonist have a single literal or conceptual destination
to reach by the end of the book? (e.g., Mario adventures)
Does your protagonist have a goal at which they and their allies can
either only succeed or fail? (e.g., saving the planet, restoring the
Republic)
Is this a mystery story wherein several possible solutions
are suggested or implied before a final reveal? (e.g., whodunit)
Is the conclusion less important than the journey? (e.g., speculative
fiction, Catcher in the Rye)

Once you've thought about those you can identify which of the following sets of conclusions you want:

A finish line; your protagonist either reaches it or dies along the
way, only to start over/back up and choose a more successful path.
(e.g., Mario adventures.)
A binary outcome of success or failure of an ultimate goal. There could be two
outcomes, or there could be multiple, not terribly significant
variations of success or of failure, perhaps plus one divergence from this. (e.g., the villain is defeated
in one of three ways, only one of which is canon for the source
novel, or the world is destroyed by a nuclear war before the pandemic
can wipe everyone out.)
Many possible, unrelated endings. (e.g., different characters could
be the perpetrator of the crime, or your character can have happy/sad
endings with different final destinations, careers, partners, powers
or ignominious failures in life.)

I hope this sets you on the right path, but there are probably many ways to get to your destination. ;)

(Disclaimer: I've never attempted to write material in this format, so this answer is all thought exercise rather than practical experience.)

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Down the Rabbit-Hole
Suppose we want to convert Alice in Wonderland to an interactive format. Which actors, items, and locations play an important role? What does a map of the world look like? What challenges did the protagonist overcome?

Player (Alice) begins the game at the Riverbank. Rabbit enters the scene.

Rabbit leaves the area, heading toward the Field. Player is supposed to follow Rabbit (but can do whatever they like).

When Player and Rabbit are both in the Field, Rabbit exposes a Hole under a hedge, previously undiscovered. Player now sees that they can enter Hole.

Player enters Hole, scores some points, falls down shaft (point of no return). Rabbit is here; he heads off to Tunnels; player can follow Rabbit through Tunnels to Hall.

That was a good warmup for the player. Now, you've got a chance to take some liberties and make things more interesting:

There were doors all round the hall, but they were all locked; and when Alice had been all the way down one side and up the other, trying every door, she walked sadly down the middle, wondering how she was ever to get out again.

Instead of a strict interpretation of the original story here, this looks like a great chance for a hub. Rather than being locked, some of the doors could open onto locations from other parts in the story. Maybe the cake that causes you to grow isn't here, and instead you need to visit the caterpillar first.
Advice from a Caterpillar
Okay, I'm not a caterpillar. But here's some advice anyway, in no particular order.
Map out every location in the game, and decide which areas are locked until some task is complete. If there are some "points of no return," those could be good places to break between chapters. Make sure that the game can't be in an "unwinnable" state at the beginning of each chapter. Your chapters (and timeline) don't need to follow the flow of the original story. Let the player work on several tasks simultaneously, within each chapter.
Let the story tell itself incrementally, through the player's actions. Let the player discover things by examining other things. Let them get stuck sometimes, and let them get "killed" (maybe Alice is rudely awoken and it was all a dream).
Alice's Evidence
After our linear intro and a non-linear hub or two, we arrive at the final trial. Here's your chance to put multiple (non-death) endings into play. All of the extra side-quests, secrets that you discovered, and out-of-the-box, non-canonical solutions to problems you figured out determine the jury's final verdict, and the game ends in victory.
"But wait, what about branching?"
I don't think we needed it. Our focus was on puzzles and progression. The bulk of the game still had a non-linear feel, even though it had a clearly-defined beginning and end. The player was allowed to explore the game world as they liked, for the most part, and interact with it through trial and error.
True branching story lines are pretty rare; there's usually basically one overall plot line, where you may get to do things in a different order, or make some choices that affect later parts of the story (good choices could unlock secrets, bad choices could make side-quests unavailable). If your goal is to make an enjoyable game without inventing an entire fictional universe, I think this approach will do the trick.

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Creating a branch is the easy part

To create a branch, as you read the book take note of every choice the character makes. Map those out - what that choice leads to, what does that in turn lead to, and so on.

Then, consider what could happen if the character chose instead to do something else instead. What happens then?

Some of those alternative choices necessarily lead to a dead end. For example, if Frodo chooses not to take the Ring from Rivendell to Mordor, there's no story. So that's a choice you don't offer the player.

Other choices offer possibilities. Frodo chooses to try to go over Caradhras rather than through Moria, maybe he has a different adventure there, with different consequences. What would that adventure and those consequences be? That's for you to decide. That's where your creating work must come in.

The hard part is getting all branches to collapse back on themselves

You don't want to be creating ten different games depending on ten different choices you offer your player. You want choices to offer some variations, but ultimately you want the story to stay more or less on the same track. For the most part, all choices should lead to the same environments and the same boss fights.

The trouble with books is that very often, in order for the character to arrive at the ending, they had to pick a specific choice at each branching point. Which is not what you want.

Therefore, right from the start, you need to mark for yourself the points in the story that are non-negotiable. Continuing with the Lord of the Rings example, perhaps the destruction of the Ring is non-negotiable, but Frodo's survival, other companions' survival, the survival of locations like Minas Tirith - those would depend on players' choices. Then, whatever choices you allow the players to make, you have to guide the results of those choices in such a way that they still lead to the non-negotiable parts, perhaps by way of the negotiable ones.

An important note: if only a very specific progression of choices leads to the optimal end, while all other paths lead to extremely non-optimal ones, players will not like you. That trope is called Guide Dang You!. (The trope also encompasses other situations when you absolutely need a guide to the game.) It follows that more than one path should lead to optimal results.

However, it's possible for more than one ending to be optimal. For example, maybe Boromir ruling Gondor isn't a worse option than Aragorn ruling Gondor.

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