The author was an Englishman of the 16th and 17th centuries. He was literate in Latin, and Greek as well as English. He is liable to have also known one or more other languages.
The author was not the sole author of the treatise. His self-perception was that he had furthered the work. This is akin to having conducted additional research in a field and being able to add one's findings. However, the culture was so different in Burton's day that the way to do this was simply to add to a written treatise on the same topic. There is something about this method that is still used, as scholars add to bodies of work, even though in other respects academic convention is very different from the ways of the 16th and 17th centuries.