Kevin;
Prior to WW2 the International Correspondence Schools home study courses made it possible for
working men to get the equivalent of a college education by studying at home. Remember, this was a time when many men went to work at an early age and many never even finished grade school. The coursers were very thorough and your lessons were graded by the schools professors and returned to you along with suggestions to help you with your studies.
Along with each section of the studies was furnished reference books that not only helped
with the studies but were a valuable reference
source in the workplace.
The courses covered just about every occupation
and on the railroads they were taken by track
laborers aiming to become section foremen, firemen
studying to become engineers,etc. The railroad management encouraged the studies and considered it a good sign that a man was interested in bettering himself. Actually several of the management people I worked under were ICS Allumni.
Finding a set nowdays is a good way to get a valuable reference library.
Jim