I just thought of one, Gaston Eysselinck. Almost nothing has been published about him in English. From what I understand, he was an important Modernist in Belgium, but died so young (under 45, I think) that his body of work was very small and mostly houses. He is most famous for his own house:
I can't think of any right now; however, if you want to know more about O'Gorman, see Builders in the Sun: Five Mexican Architects. It has about 40 or 50 pages of material on him.