400,000 books, which was pretty much every book ever written at the time. Egypt had book collectors, who would go to other countries and find books for the library. Every trading ship or caravan - customs were not looking for smuggled goods, but for books, which were copied. Centuries of oral tradition, painstakingly written down by scribes. It is painful to think what was lost.
One example - the Emperor Claudius wrote an Etruscan dictionary, by talking to the last few people who spoke it. He gifted a copy to the library, then when he was assassinated, his writings were removed. The very basis of Latin, the ideals of the Republic, and an entire culture - gone without trace.
The Baghdad library had the same philosophy. Take a copy of everything. When I die and am sitting at the gates of hell, I have one request of the Devil. To be able to beat the crap out of these people who did it for all eternity.