Is it known what was lost in the destruction of library of Alexandria?
Unfortunately, the index of the Library of Alexandria, Callimachus’ Pinakes, has been lost and so we have no knowledge of what books were contained within the library. Numbers on the amount of texts within the library vary, with some stating it at 40,000 and others at up to 400,000.
In all, it must be noted that the Library of Alexandria was not fully destroyed by one event or group but slowly fell into decline along with the fall of the Ptolemaic Dynasty and the eventual Muslim conquest of the city in the seventh century AD. What most likely happened was that texts were slowly taken or transported from the library to other places, where they were lost or decayed. It is also important to note that almost every text within the library was written on papyrus, and so any texts that were transported to Europe were almost definitely destroyed by damp and mold.
We do know from sources that the library held copies of the whole of the Epic Cycle, which was the entire story of the Trojan War (the Iliad and Odyssey being the only two that survive). We also know that all the works of the major Greek playwrights and philosophers were held there, along with a number of Eastern and Roman scholars’ works, but is impossible to know the true contents of everything that was held within the library.