Haldeman-Julius Publishing Company of Gerard, Kansas (1919-1978) was a prolific publisher of books on a wide range of subjects, including freethought, atheism, skepticism, "scandalous" topics like sex education and anarchism, revisionist history, fiction, educational texts, how-to booklets, and more. These books were mostly published in small-type, inexpensive (10 or 15 cents) pocket size booklets as the "Little Blue Books" series, and a larger "Big Blue Books" series. Haldeman-Julius experimented with selling Little Blue Books in vending machines, and many stores had a rack of Little Blue Books making them very popular.
The company was run in its heyday by Emanuel Julius, who also published the socialist Appeal to Reason newspaper. After his death in 1951, his son ran the business, although by then mainly selling backstock by mail order, until the business closed for good following a 1978 fire. However, during the 1920s and 1930s, the Little Blue Books were exceedingly popular due to their low price and brought all kinds of naughty and subversive literature to the masses - the perfect thing for geeky autodidacts living in their mom's basement in the 1920s long before there was an internet, or for the working classes to read on those long freight train rides between jobs during the Great Depression. Needless to say, a publisher like that could not stay popular in the atmosphere of moral panic that frequently overtakes the United States, and so following World War II, Haldeman-Julius lost most of their market due to pressure from J. Edgar Hoover's FBI, and retreated to being mainly a mail order business.
Some of their remaining backstock of Big Blue Books on atheistic and anti-religion topics was acquired by, and is still sold today by, the American Atheists.