non-free premium non-fiction: The Teaching Company
about $100 per course (free at some public libraries)
The Cadillac of audible non-fiction, the teaching company offers great college courses in audioform. Now that they offer content in mp3 form, they qualify as a podpook publisher.
The courses are designed for adult life long learners. This means that they strive at the parts of a subject that are intellectually interesting but not bogged down in the minutia that professional training would entail. So it may fail some looking for professional training but they are interesting enough.