Correspondence courses were at one time fairly common in chiropractic. I'm not sure when the powers-that-be decided to get rid of them, but certainly prior to WWII they were in existence. Interestingly, some of the educators in chiropractic who were most outspoken against correspondence programs were also selling them through their own colleges!
One important fact to understand is that correspondence courses were the only way to earn a chiropractic diploma by black students. Most of the chiropractic colleges discriminated against black students, so for many early black chiropractors, correspondence courses were the only way to earn an education.
Of course, learning to be a chiropractor via books and charts is virtually impossible, particularly considering the lack of being able to see videos of adjustments, even, so how these doctors learned the actual adjusting process would be interesting to see. I would love to see a full set of books and notes from such a course, but as of now, I have just a recruitment letter sent out by American University in Illinois. To put things in perspective, the $92 cash for the entire course paid up front is equal to about $1131 by today's standards, and the installment plan of $105 is $1291. A bargain by any calculation! Enjoy the photos!