This certainly is not compatible with the prisoner scenario above, but maybe it's possible to adjust the scenario without breaking many of the existing explanations.
Suppose the prisoner had or gained over the years legitimate access to some books (I feel like I'm retelling The Shawshank Redemption). Maybe then the author(s) had only to hide the fact that they were working on a book of their own, but a prison library would make it much easier not only to smuggle in some empty vellum among books, but also would make the task of hiding the manuscript more trivial. It would just be "some herbal folios" and "I have no idea who brought them or when" and "no I don't know what it says, it's some foreign language".
Maybe even there would be some legitimate reason to have some ink and a quill in the library, just for the purpose of "keeping the records of the books".
This doesn't break most of the explanations above, it's possible the work would still have to be performed on individual bifolios smuggled from the library, in the night in the cell, with a little ink and an old quill. But makes the rest of the logistics easier and allows referencing some other manuscripts after studying them during the day.
Suppose the prisoner had or gained over the years legitimate access to some books (I feel like I'm retelling The Shawshank Redemption). Maybe then the author(s) had only to hide the fact that they were working on a book of their own, but a prison library would make it much easier not only to smuggle in some empty vellum among books, but also would make the task of hiding the manuscript more trivial. It would just be "some herbal folios" and "I have no idea who brought them or when" and "no I don't know what it says, it's some foreign language".
Maybe even there would be some legitimate reason to have some ink and a quill in the library, just for the purpose of "keeping the records of the books".
This doesn't break most of the explanations above, it's possible the work would still have to be performed on individual bifolios smuggled from the library, in the night in the cell, with a little ink and an old quill. But makes the rest of the logistics easier and allows referencing some other manuscripts after studying them during the day.