Morton Gould
This one I can see to be honest, depending on how deep the book wanted to go. Although more varied than what he's best known for, Morton Gould was probably one of the leading composers at the time of what would now be called "crossover music".
Anything "crossover" or "light" is not as rigorous as the more academic stuff, but this area (and cinema and theater) were areas that always tended to remain tonal overall in the 20th century, and can produce crowd-pleasing stuff. I do feel that some interesting things can be found in this area, given the constraints of course. (I feel the same about the British "light music" too that I've heard.)
I would've included some of the ones Dundonnell mentioned (like Hanson and Piston at minimum) before Gould though! But Gould is more worthy of inclusion than PDQ Bach. :)
My guess is for a lot of that list, there was a reach for anything relatively modern. These days there probably there really aren't any "big names" of mostly tonal American concert hall composers. For relatively modern examples of American orchestral "beauty", I would've stuck with film composers personally...
