My guess is the Book of Infinite Spells. The Book of Vile Darkness is arguably similar to some other supplements, such as Lords of Darkness. I suspect the Book of Infinite Spells would contain rules and advice on creating new spells for your campaign. As each campaign is different, and there are multiple ways to create spells, the title -Book of Infinite Spells - would be appropriate.
Besides, Book of Vile Darkness would cause some publicity problems.
Of course, Zulkir or Monte could give us more of a hint. Or at least give us a better idea when they can reveal the nature of this mysterious project. (However, I do know the adage that good things come to those who wait.)