The murder of Hypatia in 415 AD by a Christian mob is reported in the Reader's Digest Book, "After Jesus: The Triumph of Christianity" page 256.
Well, there was a Hypatia, and she was murdered by a Christian mob, if one can call it that. This seems pretty much settled history with several sources.
On the other hand, there is no Library of Alexandria linked to the asassination story, and the date of the alleged Christian destruction, 391 AD, was 24 years prior to the murder, 415 AD. And Hypatia was a scholar at the Museon, a Platonic/Philosophical institution, so this apparently still existed at the time of the murder in spite of the events of 391.