Just how big is this "coffee table book?"
I can't imagine it will be larger than the largest size print you would make from a 1D or 5D. The reproduction is going to be in CMYK at about 300 DPI, right? Which means it can't possibly be more demanding than an actual print. So long as you shoot at ISO 400 or less, what advantage would larger format offer?
What am I missing here?
You're not missing a thing. I'm a Medium Format devotee, but it's generally unnecessary without a specific need for what it offers. Some will tout the "look"...but only a trained eye can spot that. I got into it because I wanted resolving power for very large prints.
I have to say, if I was the one hired for this shot, I'd be using a 5D3 and a 24mm TSE. If I wanted more resolution, I'd rent the Schneider-Kreuznach 50mm PC lens and do some stitching.
5DIII and TSE lens would be the way I would do this as well. Shift the lens from side to side and the stitch the images. You might get a panning head to help keep the stitched images in lines as well.