Like Paul Davenport I suspect that literacy was common in the 19th century and earlier. Whole books have been written about working class 'auto-didacts' in the 18th and 19th centuries. The Northamptonshire labourer, John Clare, for example had a minimal amount of education but published several volumes of poetry in his lifetime (and several more have been attributed to him after his death). In South Lancashire many districts had botanical societies made up of men who could not only read and write but were familiar with the Linnaean system of plant classification. One of them, a poor shoemaker named Richard Buxton, taught himself to read and write before embarking on a study of the science of botany and eventually publishing a local Flora (prefaced with his autobiography).

I've just been reading a biography of the 17th century herbalist, Nicholas Culpepper. He defied the patrician College of Physicians and published many of their Latin tomes in English translations. Why would he have done that if he didn't think that there was a market for them?