In the Church News a couple of weeks ago, they had an article regarding about this book. They interviewed Richard Turley and he said that it was taking much longer than they originally antipicated. They do plan on releasing the book next year, in time for the 150 anniversary of the Mountain Meadows Massacre. I was going to get the article and post it, but I've been sick the past while, so didn't get to it. It will be this weekend, I will ping you both to it.
While much is not clear about whether or not the ordering of the slaughter went straight to the top, one thing is clear: Brigham Young never felt any regret or remorse about the slaughter, until the day he died, never ordered the burying of the dead, and in fact viewed and then personally ordered the desecration of the monument to the dead put up by the US army.