Great book I ran across at the Library of Congress. It's somewhat old (1989) but IMO the one of the best movie poster books out there. It's ginormous - 342 pages, 11"x13", 6 lbs..
I ordered a copy from Amazon:
http://www.amazon.com/Reel-Art-Posters-Golden-Silver/dp/0896598691You can order it relatively cheap from the used book dealers listed on that site.
Plenty of in-depth info on the subject of how many posters were printed in the 1920s and 1930s. Turns out quite a few to service the 22,000 US theaters around in 1930, which declined to 14,000 by 1940 due to the Great Depression: