Knitting Propaganda

I know from some vintage photographs (WW2 era) that there was a big knitting for the troops effort. I don't have any idea how often these organized efforts happened, or who was organizing them. I came across the following knitting propaganda:

Photo credit Jessica Spengler via a Creative Commons License

I would love to learn more about these knitting initiatives. If you know anything about them, either from a personal perspective or a good book to turn to, please let me know through the KPOTD submission form.