@theinvisibleworm:
Say the books were burned, the evidence of their having existed in the first place would be gone.
But those volumes were collected from across the entire world; even if the people who collected them died along with the Library, they still had to get them somewhere.
If some smart (archaeologist for instance) person comes along and sees those books thrown into the lake, they'll know that the island was once a place of great scholarship, and probably investigate further since it'd be a pretty strange site to see a lake full of books.
Not really; Ohara seems to have been rather well known thanks to it's scholars even before it was annihilated. So the presence of books in the lake wouldn't really surprise anybody else that has dedicated their entire life to history.
Plus, it seems from the last few chapters that scholars across the world have been leaned on by the World Government at one time or another; Ohara was probably the last of any number of great learning centers as it's competitors were previously wiped out by the World Government.
While the books are almost certainly (with the exception of some sci-fi or supernatural explanation) illegible, the fact that the books are still in ohara is a victory.
Provided the Navy didn't have them gathered up and shredded to make confetti for a parade. Of course, they might have just collected the books to see what material was salvageable.
If so, the archaeologists saved them only to put them in the hands of the World Government.