How big is the college textbook market in the United States? What percentage of this market is new textbooks vs. used textbooks?

How big is the college textbook market in the United States? I am interested in knowing the total number of textbooks sold annually and the overall $ value of the market. Also, I’m interested to know what percentage of all college textbooks sold annually in the U.S. are new vs. used. What company/site sells the most textbooks in the U.S.?