http://www.np.reddit.com/r/askscience/comments/27p23s/what_are_the_biggest_lost_collections_of/

The usual suspects, Alexandria, Baghdad, and the Mayans show up, along with some surprising contenders, like the Nazis and Unit 731. Some highlights:

This is a reply to the (surprisingly well informed) top comment: http://np.reddit.com/r/askscience/comments/27p23s/what_are_the_biggest_lost_collections_of/ci35y8q

All this user wants to know is if the sacking of Baghdad or the destruction of Mayan writings by De Landa was a bigger loss than the Library of Alexandria. Aside from the obvious fallacy involved in measuring loss of knowledge (do I measure it in KiloSagans or Tysongrams?), he buys into the whole Library of Alexandria shtick. R5: It's been done almost to death on this sub, but here's askhistorians' discussion on the subject: http://www.reddit.com/r/AskHistorians/comments/14h7qx/how_far_did_the_destruction_of_the_library_at/

http://np.reddit.com/r/askscience/comments/27p23s/what_are_the_biggest_lost_collections_of/ci37tgd

This user claims that the records of nazi medical experiments that were burned as the war was ending would have provided incalculable benefit to doctors today. That's only true if you think that barbaric, often sadistic, experiments that served little to no medical purpose, were done in the face of every tenant of rigorous scientific study, with an objective of proving the fucked up tenants of Nazi racialist ideology, and that were highly politicized and with often falsified results, count as beneficial. I certainly don't. Unit 731 also pops its sadistic, inhumane, and useless head up to say hi.

http://np.reddit.com/r/askscience/comments/27p23s/what_are_the_biggest_lost_collections_of/ci39p6s

Ivan the Terrible's library pops up. At first, I was intrigued, because I had never heard of such a thing. However, a cursory google search reveals that it probably is not a thing. The link OP posted is from Voice of Russia, a propaganda outlet for the Russian government, and the fifth result on Google is a link to the Assassin's Creed wiki page, not the most rigorous source. In my opinion, the most telling evidence against its existence is that it doesn't even have its own wikipedia page.

These are just a few highlights, the rest of the thread is also a goldmine for your alcoholism