The way I hear it is that the proper response to your question would be "which burning of the Library of Alexandria?" Because it happened more than once. Like a lot more.
@Yuillegan: Merlin may not have known about the Oblivion War. Wizards are typically kept out of it. However, Merlin wasn't a typical wizard, and Demonreach may be an important part of the Oblivion War. I'll quote the section that makes me think that.
DEITIES AND DEMIGODS
Ancient beings once worshipped and feared. Creatures of myth and legend. More gods and demigods exist than are in your philosophies. Some are so long-lived that the world no longer knows their names, and others still walk about, free, exuding unnatural authority and asserting influence over the mortals they encounter. Too many exist to detail here or, in truth, in a bookcase full of heavy tomes, so I will warn you to take one thing to heart: these beings draw strength from worship of them, from mention of them, from the emotions they evoke. A few are encased in a prison to prevent their mischief from being again unleashed, but the rest are in this plane, in almost every nation, like nuclear warheads waiting for the best place to detonate.
That's from Dresden Files Accelerated. It's a "briefing" to Marcone from Ivy.