Harlen was both but he is "old world" magic and I guess if you were to narrow down what he accomplished best, it was teaching -- putting information into a form that was convenient to learn from.
Keep in mind that we are talking some of the basics of the industry (by today's standards) and what some might even want to call "out dated". Yet, many wonderful things come from the Tarbell course and it lays a very sound foundation in one's life, giving you a glimpse into all aspects of magic without having to invest lots of time and money to do so.
Regardless your experience level, this is something (along with the Mark Wilson Course) that should be on your book shelf at all times.
