How can the book continue until more of it is written? I seriously am having a hard time with your perspective here.
How do improv actors do improv comedy? How does an extemporaneous speech exist when the speaker only has brief notes, not explicitly written out paragraphs? How does a jazz musician compose a performance on the fly?
These are all examples of creative works that only exist in the
now. They don't take the form of a clean, collated, structured accounting of past events. Nor do they take the form of a complete construction/composition before the performance itself occurs. Instead, they draw upon prior knowledge and skills, and manifest in the actual act of performance itself. Extemporaneous speeches are particularly common, because a fully and explicitly prewritten speech can quickly become wooden, and risks falling apart if the speaker loses their place. By focusing on extemporaneous delivery, the speaker is free to respond to audience reactions and to deploy elements and information strategically.
Using the book analogy, there can be more
story (but not more "book" proper) before the book is written, because the author has extensive but not yet collated, streamlined, cleaned-up notes.
The Silmarillion existed as those notes before it was cleaned up by Christopher Tolkien, and
The Book of Lost Tales leans closer to being the raw notes, in part because some of them were too incomplete to finish.
With TTRPGs, since the experience is crafted by all the participants, the jazz example is rather more fitting IMO. Often you'll have a beginning and ending for a jazz performance where it is, if not strictly "written down," then at least rehearsed, but the middle simply
flows. Each performance will be different, with the musicians playing off one another and taking inspiration from the venue, audience, time of day/year, or whatever else tickles their fancy. That doesn't somehow mean the performance only and exclusively takes on meaning when reflecting back upon it. Indeed, many would argue that the whole
point of music is to hear it played; that it isn't a story
except when it is "now", and at all other times it isn't truly real or meaningful, because then it's just notes on a page or bumps on a disk or whatever.