Notes: According to the Oxford Classical Dictionary (3rd ed., revised), the library of Alexandria held "nearly 500,000 rolls...the equivalent, perhaps, of 100,000 modern books." The destruction of this priceless "universal library" was a catastrophic loss to history and culture, but what actually happened is apparently still unclear:
"Although there is a mythology of the burning of the Library at Alexandria, the library may have suffered several fires or acts of destruction, of varying degrees, over many years. Different cultures may have 'blamed' each other throughout history, or distanced their ancestors from responsibility, and we are left with conflicting and inconclusive fragments from ancient sources on the exact details of the destruction." Source: Wikipedia.
Caesar has been blamed for starting the fire during the civil wars, but according to "The Vanished Library" by Luciano Canfora (Chapter XV, p.82), "Strabo [the Greek geographer and historian] visited the buildings, worked in them and described them barely twenty years after Caesar's Alexandrian campaign," so it's hard to see how Caesar could have been responsible.
The Romans were blamed, the Pope, the Muslims. From what I've read so far, the people living at the time were themselves unclear about what actually happened and who was responsible. One way or another, though, the destruction of the library was an incalculable loss to human knowledge. Think about all the books the library probably contained -- the complete works of Tacitus, for instance, and who knows what else? If the library was destroyed by a single fire, it was one of the most damaging fires in human history.