It’s unfortunate that, no one knows who can use this kind of Alpha program, whether it will become a product in the future, and so on.
Even if you want to find a way to show interest to OpenAI, it can be difficult to contact them directly.
It is likely that only a select few, such as large corporations or researchers at academic institutions, have access.
If the 64K output model becomes a product in the future, it might become available as a beta version.
However, even if you can’t use the 64K output model, you can achieve your goal with creative approaches, such as generating text in segments and combining them into one book.
In fact, there are people who have published books using AI output.