So drill bits etc are in mm
afaik, I can't even buy metric drill bits, except by mail-order, out here in the hinterlands.
That wouldn't be a deal-killer for me, but I'm not even close to a "typical" American: since my hobbies include woodworking, machining, and optics, I'll often flip back and forth among fractional inches, decimal inches, and mm in the course of a day. But you'll definitely find a fair number of Americans who would avoid your book, or get annoyed after buying if they had to go through the hassle of adapting your directions to their toolkit.