Thanks for your summary and review, really enjoyed it.
And clearly this is a book many years in the making — for instance, MacAskill's BPhil thesis about moral uncertainty was published in 2000!
I had to double check that, and I think it wasn't quite that long ago, probably 2010. But on the topic of taking long, I also found this in Will's CV from ~2015-2016:
Books
Under Contract: Moral Uncertainty, Oxford University Press.
First author; co-authored with Toby Ord and Krister Bykvist. Expected publication: Feb 2017
Thanks for this! Exactly what I was looking for.