I see that there are many people bothered by the fact that the World Government didn’t bother destroying the books after the Buster Call in Ohara.
It didn’t bother me a bit because I always thought that the Marines recovered the books to be brought to other libraries, since the books are innocent, they weren’t the reason the World Government wanted to destroy Ohara. The only reason the Buster Call was releashed upon Ohara was to kill every last scholar and make an example out of Ohara’s scholars by executing them for researching the Poneglyphs:
Spandine even tries to deny he had anything to do with the Tree of Knowledge burning when the scholars freaked out about it:
We have even seen at least one book surviving the Buster Call and making it out, namely “Brag Men”.
Oda confirms that the book surved thanks to the scholar’s actions in the SBS of volume 42:
CP5 confirmed back then that no evidence of Poneglyph research has been found until they bombed open the hidden basement. There was research material related to the writings, but we’ve been shown explicitly how the agents tried to destroy the Poneglyph, so it’s very well possible that they have been tasked with destroying anything Poneglyph related on the spot – thus eliminating the threats the World Government cared about.
So conclusion: The World Government never cared about the books in the Tree of Knowledge, the Buster Call was simply to kill all the scholars and make an example out of Ohara. Even if the Marines who found the books reported it, the World Government probably didn’t care about any normal history books. It also wasn’t the goal of the scholars to protect their research about the Poneplyph, but preserving the known and written-down history that have been passed on for generations. The giants then took all the books back to Elbaf, probably because the World Government didn’t bother to save those books. And it’s also possible that they only stored important books that might be a threat to the World Government on Elbaf itself, but distributed all the other books to libraries all over the world, hence why Nami had one of the books the scholars saved back then.
And on another note, to the people who think that Dragon might be Robin’s father:
It’s heavily implied that Olvia’s husband was also a scholar who died trying to uncover more Poneglyphs.