Complex sentence: Day was national president of the American Institute of Architects, 1906--07; a founding editor of House & Garden Magazine; and author of a number of books, including American Country Houses of Today (1915).
Split into simpler sentences: Day was national president of the American Institute of Architects, 1906--07. He was a founding editor of House & Garden Magazine, and author of a number of books, including American Country Houses of Today (1915).