i was just re-reading chapter 397, and i thought of an interesting theory. Oda went to great legths to emphasize the importance of the books, the desperation of the scholors to save the books really stuck out at me as being important.
It felt unlikely that the island (and the books) wont make another apperance in the manga and i came to the conclusion that somhow all that 'history somehow survived. Ajoki's interferance in the buster call gave me the idea that he might have frozen the lake so the books would be preserved.
His expression seemed quite troubled when we see him discussing the buster call with spandin and the buddah dude.
I know it would be impossible for a lake to stay frozen for such a long time but mabey it was possible for him to do some kind of permafreeze that kept the books preserved for a very very long time.