From nathanm's post on the book:
MCAH is basically all new material. A few of the most popular recipes from MC are carried over, like mac and cheese, but even these have been re-done to be easier to make in a home kitchen.
If you already have MC, then we think of this as being like volume 6 - i.e. a volume covering home cuisine. There is very little duplication of topics between MC and MCAH - about the only ones I can think of are some coverage of sous vide technique, and some basic recipes like stocks, but even there the MCAH versions are different and adapted for the home.
I've been doing a lot of cooking from the original Modernist Cuisine set and it has resulted in some of the very best food I've ever produced, and in some cases the best I've ever eaten: so of course another volume was a no-brainer for me. It's still not cheap, but I'm pretty stoked about it. Eater has an interview with Myhrvold here with some more details. Who's in?
Edited 6/27 to add: book homepage and table of contents.
Edited by Chris Hennes, 27 June 2012 - 06:58 AM.