Александрийская библиотека уничтожалась много раз, начиная с Цезаря и кончая христианскими фанатиками. Омар библиотеку, скорее всего, не уничтожал (см. ссылку ниже). А вот французы разграбили то, что еще оставалось, и то, что уцелело, увезли в Лувр.
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The destruction of the library is attributed by some historians to a period of civil war in the late 3rd century CE -- but we know that the Museum, which was adjacent to the library, survived until the 4th century. There are also allegations dating to medieval times that claim that Caliph Omar, during an invasion in the 7th century, ordered the Library to be destroyed, but these claims are generally regarded as a Christian attack on Muslims, and include many indications of fabrication, such as the claim that the contents of the Library took six months to burn in Alexandria's public baths.
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Civil wars, decreasing investments in maintenance and acquisition of new scrolls and generally declining interest in non-religious pursuits likely contributed to a reduction in the body of material available in the Library, especially in the fourth century. The Serapeum was certainly destroyed by Theophilus in 391, and the Museum and Library may have fallen victim to the same campaign.
If indeed a Christian mob was responsible for the destruction of the Library, the question remains why Plutarch casually referred to the destruction of "the great library" by Caesar in his Life of Caesar. It is important to note that most surviving ancient works, including Plutarch, were copied throughout the Middle Ages by Christian monks. During this copying process, errors have sometimes been made, and some have argued that deliberate forgery is not out of the question, especially for politically sensitive issues. Other explanations are certainly possible, and the fate of the Library will continue to be the subject of much heated historical debate.
http://en.wikipedia.org/wiki/Library_of_Alexandria
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